Fixing NPE when animating bounds.

Bug: 36163225
Test: android.server.cts.ActivityManagerPinnedStackTests
Change-Id: I68f807f80768d279cdd55243592f87d3f752b1c6
(cherry picked from commit bb2505465586a1177d84de2faa2470f45cf95441)
diff --git a/services/core/java/com/android/server/wm/PinnedStackWindowController.java b/services/core/java/com/android/server/wm/PinnedStackWindowController.java
index 6a0e353..396f50e 100644
--- a/services/core/java/com/android/server/wm/PinnedStackWindowController.java
+++ b/services/core/java/com/android/server/wm/PinnedStackWindowController.java
@@ -64,6 +64,9 @@
             mContainer.getBounds(originalBounds);
             mContainer.setAnimatingBounds(toBounds);
             UiThread.getHandler().post(() -> {
+                if (mContainer == null) {
+                    return;
+                }
                 mService.mBoundsAnimationController.animateBounds(mContainer, originalBounds,
                         toBounds, animationDuration, moveToFullscreen);
             });