Add null check to work around async unregistration of receiver

Bug: 193890703
Test: atest SystemUITests
Change-Id: I47272513482c2ed31167574d2f2cab6094cfd590

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

@@ -1642,10 +1642,8 @@ public class NavigationBar implements View.OnAttachStateChangeListener,
     private final BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() {
         @Override
         public void onReceive(Context context, Intent intent) {
-            // This receiver is unregistered when the view is detached, but on devices with multiple
-            // displays, it can sometimes still receive an ACTION_SCREEN_ON/ACTION_SCREEN_OFF on
-            // display switch, after it was detached, so this null check ensures no crash in that
-            // scenario.
+            // TODO(193941146): Currently unregistering a receiver through BroadcastDispatcher is
+            // async, but we've already cleared the fields. Just return early in this case.
             if (mNavigationBarView == null) {
                 return;
             }