I'm trying to upgrade to Kotlin 4.1 in our Android project but it's throwing an obscure error in the kaptDebugKotlin step. Everything runs fine on kotlin version 1.3.72

I'm using:

classpath "com.android.tools.build:gradle:4.0.1"
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.4.10"

Here's the stacktrace from ./gradlew assebleDebug --stacktrace

org.gradle.api.tasks.TaskExecutionException: Execution failed for task ':app:kaptDebugKotlin'.
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.lambda$executeIfValid$1(ExecuteActionsTaskExecuter.java:205)
        at org.gradle.internal.Try$Failure.ifSuccessfulOrElse(Try.java:263)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeIfValid(ExecuteActionsTaskExecuter.java:203)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.execute(ExecuteActionsTaskExecuter.java:184)
        at org.gradle.api.internal.tasks.execution.CleanupStaleOutputsExecuter.execute(CleanupStaleOutputsExecuter.java:109)
        at org.gradle.api.internal.tasks.execution.FinalizePropertiesTaskExecuter.execute(FinalizePropertiesTaskExecuter.java:46)
        at org.gradle.api.internal.tasks.execution.ResolveTaskExecutionModeExecuter.execute(ResolveTaskExecutionModeExecuter.java:62)
        at org.gradle.api.internal.tasks.execution.SkipTaskWithNoActionsExecuter.execute(SkipTaskWithNoActionsExecuter.java:57)
        at org.gradle.api.internal.tasks.execution.SkipOnlyIfTaskExecuter.execute(SkipOnlyIfTaskExecuter.java:56)
        at org.gradle.api.internal.tasks.execution.CatchExceptionTaskExecuter.execute(CatchExceptionTaskExecuter.java:36)
        at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter$1.executeTask(EventFiringTaskExecuter.java:77)
        at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter$1.call(EventFiringTaskExecuter.java:55)
        at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter$1.call(EventFiringTaskExecuter.java:52)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$CallableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:416)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$CallableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:406)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$1.execute(DefaultBuildOperationExecutor.java:165)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:250)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:158)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.call(DefaultBuildOperationExecutor.java:102)
        at org.gradle.internal.operations.DelegatingBuildOperationExecutor.call(DelegatingBuildOperationExecutor.java:36)
        at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter.execute(EventFiringTaskExecuter.java:52)
        at org.gradle.execution.plan.LocalTaskNodeExecutor.execute(LocalTaskNodeExecutor.java:41)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$InvokeNodeExecutorsAction.execute(DefaultTaskExecutionGraph.java:372)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$InvokeNodeExecutorsAction.execute(DefaultTaskExecutionGraph.java:359)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$BuildOperationAwareExecutionAction.execute(DefaultTaskExecutionGraph.java:352)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$BuildOperationAwareExecutionAction.execute(DefaultTaskExecutionGraph.java:338)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker.lambda$run$0(DefaultPlanExecutor.java:127)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker.execute(DefaultPlanExecutor.java:191)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker.executeNextNode(DefaultPlanExecutor.java:182)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker.run(DefaultPlanExecutor.java:124)
        at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:64)
        at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:48)
        at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
Caused by: org.gradle.workers.internal.DefaultWorkerExecutor$WorkExecutionException: A failure occurred while executing org.jetbrains.kotlin.gradle.internal.KaptExecution
        at org.gradle.workers.internal.DefaultWorkerExecutor$WorkItemExecution.waitForCompletion(DefaultWorkerExecutor.java:354)
        at org.gradle.internal.work.DefaultAsyncWorkTracker.waitForItemsAndGatherFailures(DefaultAsyncWorkTracker.java:142)
        at org.gradle.internal.work.DefaultAsyncWorkTracker.access$000(DefaultAsyncWorkTracker.java:34)
        at org.gradle.internal.work.DefaultAsyncWorkTracker$1.run(DefaultAsyncWorkTracker.java:106)
        at org.gradle.internal.Factories$1.create(Factories.java:26)
        at org.gradle.internal.work.DefaultWorkerLeaseService.withoutLocks(DefaultWorkerLeaseService.java:260)
        at org.gradle.internal.work.DefaultWorkerLeaseService.withoutProjectLock(DefaultWorkerLeaseService.java:171)
        at org.gradle.internal.work.DefaultWorkerLeaseService.withoutProjectLock(DefaultWorkerLeaseService.java:165)
        at org.gradle.internal.work.StopShieldingWorkerLeaseService.withoutProjectLock(StopShieldingWorkerLeaseService.java:95)
        at org.gradle.internal.work.DefaultAsyncWorkTracker.waitForItemsAndGatherFailures(DefaultAsyncWorkTracker.java:102)
        at org.gradle.internal.work.DefaultAsyncWorkTracker.waitForAll(DefaultAsyncWorkTracker.java:80)
        at org.gradle.internal.work.DefaultAsyncWorkTracker.waitForCompletion(DefaultAsyncWorkTracker.java:68)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter$3.run(ExecuteActionsTaskExecuter.java:576)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:402)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:394)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$1.execute(DefaultBuildOperationExecutor.java:165)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:250)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:158)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:92)
        at org.gradle.internal.operations.DelegatingBuildOperationExecutor.run(DelegatingBuildOperationExecutor.java:31)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeAction(ExecuteActionsTaskExecuter.java:553)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeActions(ExecuteActionsTaskExecuter.java:536)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.access$300(ExecuteActionsTaskExecuter.java:109)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter$TaskExecution.executeWithPreviousOutputFiles(ExecuteActionsTaskExecuter.java:276)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter$TaskExecution.execute(ExecuteActionsTaskExecuter.java:265)
        at org.gradle.internal.execution.steps.ExecuteStep.lambda$execute$0(ExecuteStep.java:32)
        at org.gradle.internal.execution.steps.ExecuteStep.execute(ExecuteStep.java:32)
        at org.gradle.internal.execution.steps.ExecuteStep.execute(ExecuteStep.java:26)
        at org.gradle.internal.execution.steps.CleanupOutputsStep.execute(CleanupOutputsStep.java:63)
        at org.gradle.internal.execution.steps.CleanupOutputsStep.execute(CleanupOutputsStep.java:35)
        at org.gradle.internal.execution.steps.ResolveInputChangesStep.execute(ResolveInputChangesStep.java:49)
        at org.gradle.internal.execution.steps.ResolveInputChangesStep.execute(ResolveInputChangesStep.java:34)
        at org.gradle.internal.execution.steps.CancelExecutionStep.execute(CancelExecutionStep.java:43)
        at org.gradle.internal.execution.steps.TimeoutStep.executeWithoutTimeout(TimeoutStep.java:73)
        at org.gradle.internal.execution.steps.TimeoutStep.execute(TimeoutStep.java:54)
        at org.gradle.internal.execution.steps.CatchExceptionStep.execute(CatchExceptionStep.java:34)
        at org.gradle.internal.execution.steps.CreateOutputsStep.execute(CreateOutputsStep.java:44)
        at org.gradle.internal.execution.steps.SnapshotOutputsStep.execute(SnapshotOutputsStep.java:54)
        at org.gradle.internal.execution.steps.SnapshotOutputsStep.execute(SnapshotOutputsStep.java:38)
        at org.gradle.internal.execution.steps.BroadcastChangingOutputsStep.execute(BroadcastChangingOutputsStep.java:49)
        at org.gradle.internal.execution.steps.CacheStep.executeWithoutCache(CacheStep.java:153)
        at org.gradle.internal.execution.steps.CacheStep.execute(CacheStep.java:67)
        at org.gradle.internal.execution.steps.CacheStep.execute(CacheStep.java:41)
        at org.gradle.internal.execution.steps.StoreExecutionStateStep.execute(StoreExecutionStateStep.java:44)
        at org.gradle.internal.execution.steps.StoreExecutionStateStep.execute(StoreExecutionStateStep.java:33)
        at org.gradle.internal.execution.steps.RecordOutputsStep.execute(RecordOutputsStep.java:38)
        at org.gradle.internal.execution.steps.RecordOutputsStep.execute(RecordOutputsStep.java:24)
        at org.gradle.internal.execution.steps.SkipUpToDateStep.executeBecause(SkipUpToDateStep.java:92)
        at org.gradle.internal.execution.steps.SkipUpToDateStep.lambda$execute$0(SkipUpToDateStep.java:85)
        at org.gradle.internal.execution.steps.SkipUpToDateStep.execute(SkipUpToDateStep.java:55)
        at org.gradle.internal.execution.steps.SkipUpToDateStep.execute(SkipUpToDateStep.java:39)
        at org.gradle.internal.execution.steps.ResolveChangesStep.execute(ResolveChangesStep.java:76)
        at org.gradle.internal.execution.steps.ResolveChangesStep.execute(ResolveChangesStep.java:37)
        at org.gradle.internal.execution.steps.legacy.MarkSnapshottingInputsFinishedStep.execute(MarkSnapshottingInputsFinishedStep.java:36)
        at org.gradle.internal.execution.steps.legacy.MarkSnapshottingInputsFinishedStep.execute(MarkSnapshottingInputsFinishedStep.java:26)
        at org.gradle.internal.execution.steps.ResolveCachingStateStep.execute(ResolveCachingStateStep.java:94)
        at org.gradle.internal.execution.steps.ResolveCachingStateStep.execute(ResolveCachingStateStep.java:49)
        at org.gradle.internal.execution.steps.CaptureStateBeforeExecutionStep.execute(CaptureStateBeforeExecutionStep.java:79)
        at org.gradle.internal.execution.steps.CaptureStateBeforeExecutionStep.execute(CaptureStateBeforeExecutionStep.java:53)
        at org.gradle.internal.execution.steps.ValidateStep.execute(ValidateStep.java:74)
        at org.gradle.internal.execution.steps.SkipEmptyWorkStep.lambda$execute$2(SkipEmptyWorkStep.java:78)
        at org.gradle.internal.execution.steps.SkipEmptyWorkStep.execute(SkipEmptyWorkStep.java:78)
        at org.gradle.internal.execution.steps.SkipEmptyWorkStep.execute(SkipEmptyWorkStep.java:34)
        at org.gradle.internal.execution.steps.legacy.MarkSnapshottingInputsStartedStep.execute(MarkSnapshottingInputsStartedStep.java:39)
        at org.gradle.internal.execution.steps.LoadExecutionStateStep.execute(LoadExecutionStateStep.java:40)
        at org.gradle.internal.execution.steps.LoadExecutionStateStep.execute(LoadExecutionStateStep.java:28)
        at org.gradle.internal.execution.impl.DefaultWorkExecutor.execute(DefaultWorkExecutor.java:33)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeIfValid(ExecuteActionsTaskExecuter.java:192)
        ... 30 more
Caused by: java.lang.reflect.InvocationTargetException
        at org.jetbrains.kotlin.gradle.internal.KaptExecution.run(KaptWithoutKotlincTask.kt:158)
        at org.gradle.workers.internal.AdapterWorkAction.execute(AdapterWorkAction.java:50)
        at org.gradle.workers.internal.DefaultWorkerServer.execute(DefaultWorkerServer.java:50)
        at org.gradle.workers.internal.NoIsolationWorkerFactory$1$1.create(NoIsolationWorkerFactory.java:63)
        at org.gradle.workers.internal.NoIsolationWorkerFactory$1$1.create(NoIsolationWorkerFactory.java:59)
        at org.gradle.internal.classloader.ClassLoaderUtils.executeInClassloader(ClassLoaderUtils.java:98)
        at org.gradle.workers.internal.NoIsolationWorkerFactory$1.lambda$execute$0(NoIsolationWorkerFactory.java:59)
        at org.gradle.workers.internal.AbstractWorker$1.call(AbstractWorker.java:44)
        at org.gradle.workers.internal.AbstractWorker$1.call(AbstractWorker.java:41)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$CallableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:416)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$CallableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:406)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$1.execute(DefaultBuildOperationExecutor.java:165)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:250)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:158)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.call(DefaultBuildOperationExecutor.java:102)
        at org.gradle.internal.operations.DelegatingBuildOperationExecutor.call(DelegatingBuildOperationExecutor.java:36)
        at org.gradle.workers.internal.AbstractWorker.executeWrappedInBuildOperation(AbstractWorker.java:41)
        at org.gradle.workers.internal.NoIsolationWorkerFactory$1.execute(NoIsolationWorkerFactory.java:53)
        at org.gradle.workers.internal.DefaultWorkerExecutor.lambda$submitWork$2(DefaultWorkerExecutor.java:200)
        at org.gradle.internal.work.DefaultConditionalExecutionQueue$ExecutionRunner.runExecution(DefaultConditionalExecutionQueue.java:215)
        at org.gradle.internal.work.DefaultConditionalExecutionQueue$ExecutionRunner.runBatch(DefaultConditionalExecutionQueue.java:164)
        at org.gradle.internal.work.DefaultConditionalExecutionQueue$ExecutionRunner.run(DefaultConditionalExecutionQueue.java:131)
        ... 3 more
Caused by: org.jetbrains.kotlin.kapt3.base.util.KaptBaseError: Exception while annotation processing
        at org.jetbrains.kotlin.kapt3.base.AnnotationProcessingKt.doAnnotationProcessing(annotationProcessing.kt:83)
        at org.jetbrains.kotlin.kapt3.base.AnnotationProcessingKt.doAnnotationProcessing$default(annotationProcessing.kt:34)
        at org.jetbrains.kotlin.kapt3.base.Kapt.kapt(Kapt.kt:45)
        ... 25 more
Caused by: kotlin.KotlinNullPointerException
        at me.eugeniomarletti.kotlin.metadata.KotlinClassMetadata$data$2.invoke(KotlinMetadata.kt:55)
        at me.eugeniomarletti.kotlin.metadata.KotlinClassMetadata$data$2.invoke(KotlinMetadata.kt:53)
        at kotlin.SynchronizedLazyImpl.getValue(LazyJVM.kt:74)
        at me.eugeniomarletti.kotlin.metadata.KotlinClassMetadata.getData(KotlinMetadata.kt)
        at com.squareup.moshi.kotlin.codegen.TargetType$Companion.get(TargetType.kt:72)
        at com.squareup.moshi.kotlin.codegen.JsonClassCodegenProcessor.adapterGenerator(JsonClassCodegenProcessor.kt:97)
        at com.squareup.moshi.kotlin.codegen.JsonClassCodegenProcessor.process(JsonClassCodegenProcessor.kt:88)
        at org.jetbrains.kotlin.kapt3.base.incremental.IncrementalProcessor.process(incrementalProcessors.kt)
        at org.jetbrains.kotlin.kapt3.base.ProcessorWrapper.process(annotationProcessing.kt:161)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.callProcessor(JavacProcessingEnvironment.java:802)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.discoverAndRunProcs(JavacProcessingEnvironment.java:713)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.access$1800(JavacProcessingEnvironment.java:91)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.run(JavacProcessingEnvironment.java:1043)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.doProcessing(JavacProcessingEnvironment.java:1184)
        at com.sun.tools.javac.main.JavaCompiler.processAnnotations(JavaCompiler.java:1170)
        at com.sun.tools.javac.main.JavaCompiler.processAnnotations(JavaCompiler.java:1068)
        at org.jetbrains.kotlin.kapt3.base.AnnotationProcessingKt.doAnnotationProcessing(annotationProcessing.kt:78)
        ... 27 more

Not sure where to start looking. Happy to provide any additional details if required.

Ugh, it's telling me that my post is mostly code and I need to provide more details. So this is just filler to hopefully get around that check as I've added all the useful details I can think of.