I migrate my application on AndroidX, I fix many problem resulting, until I can build succesly my project. But when I launch my application(success) and try to use it, I have some bug and the app close. I searched the errors on the logcat and saw that they were on code I had not changed, so I can't understand why new errors appears on old code. These errors are

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context pack.myrhs.MobileRHS.getApplicationContext()' on a null object

I know that was because I call getApplicationContext on a null object but I can't see why this object is null now and not before. Cause I change nothing on the activity where it append. Can someone explains me why ?