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For googlers: I created an internal document describe the bug, after the re-entry scenario, the ChildHelper state is inconsistent, causing a follow up findViewByPosition() returns null, and NullPointerException is thrown.

https://docs.google.com/document/d/1x0h7EH3RdZX0aAzSfttf9mTKfV_-xW6ugrftuGGz3Tc/edit?usp=sharing&resourcekey=0-EybDeQPMRqn0Vaqkz3yqrw

This stacks shows how the re-enter happens (line # is inaccurate since I added a lot of println in the library code)

	at androidx.recyclerview.widget.RecyclerView$5.removeViewAt(RecyclerView.java:999)
 	at androidx.recyclerview.widget.ChildHelper.removeViewIfHidden(ChildHelper.java:393)
 	at androidx.recyclerview.widget.RecyclerView.removeAnimatingView(RecyclerView.java:1620)
 	at androidx.recyclerview.widget.RecyclerView$ItemAnimatorRestoreListener.onAnimationFinished(RecyclerView.java:13828)
 	at androidx.recyclerview.widget.RecyclerView$ItemAnimator.dispatchAnimationFinished(RecyclerView.java:14330)
 	at androidx.recyclerview.widget.SimpleItemAnimator.dispatchMoveFinished(SimpleItemAnimator.java:301)
 	at androidx.recyclerview.widget.DefaultItemAnimator$6.onAnimationEnd(DefaultItemAnimator.java:316)
 	at android.view.ViewPropertyAnimator$AnimatorEventListener.onAnimationEnd(ViewPropertyAnimator.java:1112)
 	at android.animation.Animator$AnimatorListener.onAnimationEnd(Animator.java:555)
 	at android.animation.ValueAnimator.endAnimation(ValueAnimator.java:1250)
 	at android.animation.ValueAnimator.cancel(ValueAnimator.java:1135)
 	at android.view.ViewPropertyAnimator.cancel(ViewPropertyAnimator.java:425)
 	at androidx.leanback.widget.Presenter.cancelAnimationsRecursive(Presenter.java:191)
 	at androidx.leanback.widget.Presenter.onViewDetachedFromWindow(Presenter.java:183)
 	at androidx.leanback.widget.ItemBridgeAdapter.onViewDetachedFromWindow(ItemBridgeAdapter.java:460)
 	at androidx.recyclerview.widget.RecyclerView.dispatchChildDetached(RecyclerView.java:8466)
 	at androidx.recyclerview.widget.RecyclerView$5.removeViewAt(RecyclerView.java:988)
 	at androidx.recyclerview.widget.ChildHelper.removeView(ChildHelper.java:151)
 	at androidx.recyclerview.widget.RecyclerView$LayoutManager.removeView(RecyclerView.java:9569)
 	at androidx.recyclerview.widget.RecyclerView$LayoutManager.removeAndRecycleView(RecyclerView.java:9842)

Submitted a leanback test shows the crash steps. "setSelectedPosition(0)" removes the the item "6" that is currently sliding into the RecyclerView, it causes re-entering ChildHelper.removeViewIfHidden("6") in the middle of ChildHelper.removeView("6"). After that, ChildHelper is in a bad state for the getChildCount(), the following "setSelectedPosition(2)" throws NullPointerException.

https://cs.android.com/androidx/platform/frameworks/support/+/d0866d321132bd36f828e1e39a49ca7dad83fd2c:leanback/leanback/src/androidTest/java/androidx/leanback/widget/GridWidgetTest.java;l=2252

        // showing 0 1(selected) 2 3 4 5(peek)
        setSelectedPosition(1);

        // swap 5 and 6, showing 0 1(selected) 2 3 4 6(slide in) 5(slide out)
        mActivityTestRule.runOnUiThread(() ->
                adapter.setItems(TestPresenter.generateItems(new int[]{0, 1, 2, 3, 4, 6, 5, 7, 8}),
                        TestPresenter.DIFF_CALLBACK));

        // Wait a little bit for the ItemAnimation to be started
        waitForItemAnimationStart();

        // Scroll to 0 to remove "6", this can break ChildHelper and scroll to 2 may failed to find
        // the View and crash.
        setSelectedPosition(0);
        setSelectedPosition(2);

Will be released with next alpha release, barring anything unexpected.

The following release(s) address this bug.It is possible this bug has only been partially addressed:

• androidx.leanback:leanback:1.2.0-alpha03

The following release(s) address this bug.It is possible this bug has only been partially addressed:

• androidx.recyclerview:recyclerview:1.4.0-alpha01
• androidx.recyclerview:recyclerview:1.3.2