Guard potential null case in FullscreenTaskListener.releaseWindowDecor

found in testing.

Bug: 244333148
Test: Test no longer fails.
Change-Id: Ie65ac68f11afc6777e2c9c18f8ad8386ae43082f

libs/WindowManager/Shell/src/com/android/wm/shell/fullscreen/FullscreenTaskListener.java

@@ -287,6 +287,9 @@ public class FullscreenTaskListener<T extends AutoCloseable>
     }
 
     private void releaseWindowDecor(T windowDecor) {
+        if (windowDecor == null) {
+            return;
+        }
         try {
             windowDecor.close();
         } catch (Exception e) {