Follow-up for I01e81b9cd11886da734da671c68d5732aa51009f

Test: Open app, press recents, make sure thumbnail is up-to-date
Bug: 31339431
Change-Id: I933ad05dbc46b7b71572b09d698524a9699c0fbf

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

@@ -637,6 +637,7 @@ class Task extends WindowContainer<AppWindowToken> implements DimLayer.DimLayerU
         return mFillsParent || !StackId.isTaskResizeAllowed(mStack.mStackId);
     }
 
+    @Override
     TaskWindowContainerController getController() {
         return (TaskWindowContainerController) super.getController();
     }

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

@@ -52,7 +52,9 @@ public class TaskWindowContainerController
         public void handleMessage(Message msg) {
             switch (msg.what) {
                 case REPORT_SNAPSHOT_CHANGED:
-                    mListener.onSnapshotChanged((TaskSnapshot) msg.obj);
+                    if (mListener != null) {
+                        mListener.onSnapshotChanged((TaskSnapshot) msg.obj);
+                    }
                     break;
             }
         }