Null reference to activity as runnable back handler

Bug: 302551868
Change-Id: Ia21c5696ef58c113a8d038cb0f8dfef413d991de
diff --git a/quickstep/src/com/android/quickstep/util/SplitSelectStateController.java b/quickstep/src/com/android/quickstep/util/SplitSelectStateController.java
index 0c4fcf9..9041399 100644
--- a/quickstep/src/com/android/quickstep/util/SplitSelectStateController.java
+++ b/quickstep/src/com/android/quickstep/util/SplitSelectStateController.java
@@ -118,7 +118,7 @@
     private final Handler mHandler;
     private final RecentsModel mRecentTasksModel;
     @Nullable
-    private final Runnable mActivityBackCallback;
+    private Runnable mActivityBackCallback;
     private final SplitAnimationController mSplitAnimationController;
     private final AppPairsController mAppPairsController;
     private final SplitSelectDataHolder mSplitSelectDataHolder;
@@ -182,6 +182,7 @@
 
     public void onDestroy() {
         mContext = null;
+        mActivityBackCallback = null;
     }
 
     /**