Null out mSplitHiddenTaskView when resetting split selection state

* May have accidentally been removed in ag/19984439

Fixes: 248153819
Test: Repro in bug doesn't occur
Change-Id: Icdc04de84c28963d36368f9672baf5993cd87d8a
diff --git a/quickstep/src/com/android/quickstep/views/RecentsView.java b/quickstep/src/com/android/quickstep/views/RecentsView.java
index 96607a0..b7d9432 100644
--- a/quickstep/src/com/android/quickstep/views/RecentsView.java
+++ b/quickstep/src/com/android/quickstep/views/RecentsView.java
@@ -4259,6 +4259,10 @@
         onLayout(false /*  changed */, getLeft(), getTop(), getRight(), getBottom());
         resetTaskVisuals();
         mSplitHiddenTaskViewIndex = -1;
+        if (mSplitHiddenTaskView != null) {
+            mSplitHiddenTaskView.setThumbnailVisibility(VISIBLE);
+            mSplitHiddenTaskView = null;
+        }
     }
 
     private void safeRemoveDragLayerView(@Nullable View viewToRemove) {