Fix NPE when calling updateStateLw

In multi-display environment, the existance of status bar and navigation
bar may differ per disply. In other words, we may not have status bar or
navigation bar on a external displays. Add null check to prevent this
NPE happens.

Test: WmTests
Fixes: 120779126

Change-Id: I805a061733cdbe09c5c1941a0f44e2e49964671a

services/core/java/com/android/server/wm/BarController.java

@@ -219,7 +219,7 @@ public class BarController {
     }
 
     private boolean updateStateLw(final int state) {
-        if (state != mState) {
+        if (mWin != null && state != mState) {
             mState = state;
             if (DEBUG) Slog.d(mTag, "mState: " + StatusBarManager.windowStateToString(state));
             mHandler.post(new Runnable() {