Fix null-pointer exception in all set activity first reveal animation

Fixes: 259296310
Test: started all set activity and started the first reveal animation
Change-Id: I371f62be81e7f28ce5ecee150abb5d294bb2264d

quickstep/src/com/android/launcher3/taskbar/TaskbarViewController.java

@@ -240,6 +240,7 @@ public class TaskbarViewController implements TaskbarControllers.LoggableTaskbar
         return mTaskbarView.getIconViews();
     }
 
+    @Nullable
     public View getAllAppsButtonView() {
         return mTaskbarView.getAllAppsButtonView();
     }