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/TaskbarView.java

@@ -422,6 +422,7 @@ public class TaskbarView extends FrameLayout implements FolderIcon.FolderIconPar
     /**
      * Returns the all apps button in the taskbar.
      */
+    @Nullable
     public View getAllAppsButtonView() {
         return mAllAppsButton;
     }