Fix NPE with overflow expanded view

I made it so that the overflow expanded view could return null and
didn't protect this access of it. Fix is to check if the view is
null before doing something to it.

Test: treehugger
Bug: 219491953
Change-Id: I380584223ad56c754cf59c63e3eb2131c3275154

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

@@ -1237,7 +1237,7 @@ public class BubbleStackView extends FrameLayout
                 b.getExpandedView().updateFontSize();
             }
         }
-        if (mBubbleOverflow != null) {
+        if (mBubbleOverflow != null && mBubbleOverflow.getExpandedView() != null) {
             mBubbleOverflow.getExpandedView().updateFontSize();
         }
     }