DO NOT MERGE Add logging for when IOnBackInvokedCallback is null. am: 306150025c

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/22442774

Change-Id: I8dbcbae1b0740ce5e01e86ff1a117af4f243203f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

core/java/android/window/WindowOnBackInvokedDispatcher.java

@@ -291,6 +291,7 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
                 mProgressAnimator.reset();
                 final OnBackInvokedCallback callback = mCallback.get();
                 if (callback == null) {
+                    Log.d(TAG, "Trying to call onBackInvoked() on a null callback reference.");
                     return;
                 }
                 callback.onBackInvoked();