Skip updating nav bar if it was destroyed

- Since device config updates are posted, there can be
  a queued update after we remove the nav bar, which
  will reference a null bar

Bug: 210713030
Test: Haven't been able to trigger the actual NPE
Change-Id: I05f1cb6e2a97808ad05b5aaaa046689df04995e1

packages/SystemUI/src/com/android/systemui/navigationbar/NavigationBar.java

@@ -440,7 +440,9 @@ public class NavigationBar implements View.OnAttachStateChangeListener,
                         mHomeButtonLongPressDurationMs = Optional.of(
                             properties.getLong(HOME_BUTTON_LONG_PRESS_DURATION_MS, 0)
                         ).filter(duration -> duration != 0);
-                        reconfigureHomeLongClick();
+                        if (mNavigationBarView != null) {
+                            reconfigureHomeLongClick();
+                        }
                     }
                 }
             };