Fix possible NPE

No user report of this but found in CTS.

I haven't been able to repro this, I think the
expanded view needs to be animating when the bubble
is removed but I haven't been able to reproduce this
via the unit tests or manually to confirm.

Bug: 200603814
Test: atest NotificationManagerTest watch logs and make sure there
      isn't an exception
Change-Id: I70c286a67410d4cd6bbd272a4f27562a5ae1cb99

libs/WindowManager/Shell/src/com/android/wm/shell/bubbles/BubbleStackView.java

@@ -968,7 +968,7 @@ public class BubbleStackView extends FrameLayout
             }
         });
         mExpandedViewAlphaAnimator.addUpdateListener(valueAnimator -> {
-            if (mExpandedBubble != null) {
+            if (mExpandedBubble != null && mExpandedBubble.getExpandedView() != null) {
                 mExpandedBubble.getExpandedView().setTaskViewAlpha(
                         (float) valueAnimator.getAnimatedValue());
             }