Fixing NPE when animating bounds.

Bug: 36163225
Test: android.server.cts.ActivityManagerPinnedStackTests
Change-Id: I68f807f80768d279cdd55243592f87d3f752b1c6

services/core/java/com/android/server/wm/PinnedStackWindowController.java

@@ -64,6 +64,9 @@ public class PinnedStackWindowController extends StackWindowController {
             mContainer.getBounds(originalBounds);
             mContainer.setAnimatingBounds(toBounds);
             UiThread.getHandler().post(() -> {
+                if (mContainer == null) {
+                    return;
+                }
                 mService.mBoundsAnimationController.animateBounds(mContainer, originalBounds,
                         toBounds, animationDuration, moveToFullscreen);
             });