Check your Gradle scripts root. If there is this file: gradle.properties (Global Properties), check that values are correct.

In my case, I removed all these properties and my problem solved

systemProp.http.proxyHost=
systemProp.http.proxyPort=80
systemProp.https.proxyHost=
systemProp.https.proxyPort=80

It may be problem with uploadMappingFile

Set the firebaseCrashlytics.enableMappingFileUpload Gradle extension property to true in your app-level build.gradle file.

// To enable Crashlytics mapping file upload for specific product flavors:
  flavorDimensions "environment"
  productFlavors {
    staging {
      dimension "environment"
      ...
      firebaseCrashlytics {
        mappingFileUploadEnabled false
      }
    }
    prod {
      dimension "environment"
      ...
      firebaseCrashlytics {
        mappingFileUploadEnabled true
      }
    }
  }

for more details visit here

I got the same issue, I fixed that issue by applying Proguard rules,

https://firebase.google.com/docs/crashlytics/get-deobfuscated-reports?platform=android

In project proguard-rules.pro

-keepattributes SourceFile,LineNumberTable
-keep public class * extends java.lang.Exception

Gradle

firebaseCrashlytics { mappingFileUploadEnabled true }

Yep, problem started at AS 4.0 up.. Here are the clearer fixes... (Cos Some may think the "gradle.properties" file being talked about here is what is in the AS project instead of the global one.

Delete the following lines from the "gradle.propeties" file on your PC gradle installation location

systemProp.http.proxyHost= systemProp.https.proxyHost= systemProp.https.proxyPort=80 systemProp.http.proxyPort=80

as seen in n my picture below here is a typical file location of this gradle.propeties file

Now click rebuild in your project and VIOLA... Problem solved

This issue is caused due to empty xml file in your res directory. Open the res directory in explorer/finder sort all the xml according to their size. Find the xml with 0kb / empty xml and delete it. Now you can create your signed bundled apk.