Add Null checks to NavigationBarView

An apparent race condition means that the NavigationBarView tries to
use the NotificationPanelViewController before the StatusBar has
had an opportunity to create it.

Fixes: 156242260
Test: atest SystemUITests
Change-Id: I588fca9eeb3121cab30d714b1c1a55397d2f0da5

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

@@ -822,7 +822,7 @@ public class NavigationBarView extends FrameLayout implements
      */
     public void updateSlippery() {
         setSlippery(!isQuickStepSwipeUpEnabled() ||
-                (mPanelView.isFullyExpanded() && !mPanelView.isCollapsing()));
+                (mPanelView != null && mPanelView.isFullyExpanded() && !mPanelView.isCollapsing()));
     }
 
     private void setSlippery(boolean slippery) {