Migrate to AndroidX

[Redirion]

• I carefully read the contribution guidelines and agree to them.

Third attempt.
This PR supersedes PR #2598 (seems to be no longer maintained).
This was finally possible after PR #2535 was merged.
I took the stable approach for continued usage of the modified FlingBehavior.java from this commit 0d25645

Closes #2641

Debug apk: https://github.com/Redirion/NewPipe/releases/download/v0.17.4/app-debug.apk

Note that I left everything unchanged, the Extractor still has the NullPointerException on getChannelCount.

[snappyapple632]

Tested the APK for a couple of days now, no issues on my end.

[EmmanuelMess]

This PR supersedes PR #2598 (seems to be no longer maintained).

Very sorry about that.

[mauriciocolli]

Good to merge!

[mauriciocolli]

Oh, just noticed that I don't have the permissions to approve pull requests anymore (was inactive for some time), it is good to merge though.

[TobiGr]

Thanks for taking on this again!
The migration even brings some layout fixes for KitKat ❤️

[TobiGr]

@Redirion Below is the exception from #2697. I was able to reproduce it. It happens when rotating the screen on a channel page twice. E.g from portrait to landscape and back again. I guess this is what I did when opening the video and rotating the phone to view a video in landscape. However, I tried this with commits before this PR and did produce the crash. Would be good if you can take a look at it.

Exception

• User Action: ui error
• Request: App crash, UI failure
• Content Language: GB
• Service: none
• Version: 0.17.3
• OS: Linux ,release-keys 6.0 - 23

Crash log

java.lang.RuntimeException: Unable to destroy activity {org.schabi.newpipe.debug/org.schabi.newpipe.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference
	at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3852)
	at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:3870)
	at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4074)
	at android.app.ActivityThread.-wrap15(ActivityThread.java)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1360)
	at android.os.Handler.dispatchMessage(Handler.java:102)
	at android.os.Looper.loop(Looper.java:148)
	at android.app.ActivityThread.main(ActivityThread.java:5443)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference
	at org.schabi.newpipe.fragments.MainFragment.onDestroy(MainFragment.java:115)
	at androidx.fragment.app.Fragment.performDestroy(Fragment.java:2830)
	at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1028)
	at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
	at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1310)
	at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2659)
	at androidx.fragment.app.FragmentManagerImpl.dispatchDestroy(FragmentManagerImpl.java:2644)
	at androidx.fragment.app.FragmentController.dispatchDestroy(FragmentController.java:329)
	at androidx.fragment.app.FragmentActivity.onDestroy(FragmentActivity.java:366)
	at androidx.appcompat.app.AppCompatActivity.onDestroy(AppCompatActivity.java:233)
	at org.schabi.newpipe.MainActivity.onDestroy(MainActivity.java:352)
	at android.app.Activity.performDestroy(Activity.java:6415)
	at android.app.Instrumentation.callActivityOnDestroy(Instrumentation.java:1165)
	at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3839)
	... 10 more
java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference
	at org.schabi.newpipe.fragments.MainFragment.onDestroy(MainFragment.java:115)
	at androidx.fragment.app.Fragment.performDestroy(Fragment.java:2830)
	at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1028)
	at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
	at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1310)
	at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2659)
	at androidx.fragment.app.FragmentManagerImpl.dispatchDestroy(FragmentManagerImpl.java:2644)
	at androidx.fragment.app.FragmentController.dispatchDestroy(FragmentController.java:329)
	at androidx.fragment.app.FragmentActivity.onDestroy(FragmentActivity.java:366)
	at androidx.appcompat.app.AppCompatActivity.onDestroy(AppCompatActivity.java:233)
	at org.schabi.newpipe.MainActivity.onDestroy(MainActivity.java:352)
	at android.app.Activity.performDestroy(Activity.java:6415)
	at android.app.Instrumentation.callActivityOnDestroy(Instrumentation.java:1165)
	at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3839)
	at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:3870)
	at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4074)
	at android.app.ActivityThread.-wrap15(ActivityThread.java)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1360)
	at android.os.Handler.dispatchMessage(Handler.java:102)
	at android.os.Looper.loop(Looper.java:148)
	at android.app.ActivityThread.main(ActivityThread.java:5443)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

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[Redirion]

It might be easily mitigated with a null check but fixing the root cause which looks like a race condition between destroying and recreating the views. I will have a look tomorrow morning

[Redirion]

I went ahead with the simple fix. There was no obvious root cause and nulling the adapter of the ViewPager to clean it up is the correct thing to do. So I just added the null check inline with the other Fragment classes.