Add null check on NPVC

QS may be null at some points (when the fragment is being recreated).
Use a null check to prevent NPE.

Test: no test
Fixes: 189701093
Change-Id: I393b63aedc2fd58c079523a05778411c178caa22

packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelViewController.java

@@ -2519,7 +2519,9 @@ public class NotificationPanelViewController extends PanelViewController {
                 break;
             case FLING_HIDE:
             default:
-                mQs.closeDetail();
+                if (mQs != null) {
+                    mQs.closeDetail();
+                }
                 target = 0;
         }
         if (target == mQsExpansionHeight) {