It turns out that the refactor tool in Android Studio Refactor > Migrate to AndroidX didn’t correctly migrate the XML for the BottomSheetBehaviour. The old location was android.support.design.widget.BottomSheetBehavior, and was not modified by the migration tool. The original XML was: <fragment android:id=”@+id/player_bottom_sheet_fragment” android:name=”app.rxsongbrowsertrials.ui.player.PlayerToggleFragment” android:layout_width=”match_parent” android:layout_height=”match_parent” app:behavior_hideable=”false” app:behavior_peekHeight=”56dp” app:layout_behavior=”android.support.design.widget.BottomSheetBehavior” /> The new location is com.google.android.material.bottomsheet.BottomSheetBehavior, so … Read more