Fix crash when the app is closed while playing 360 videos

app/src/common/shared/org/mozilla/vrbrowser/VRBrowserActivity.java

@@ -395,8 +395,6 @@ protected void onDestroy() {
         }
 
         // Remove all widget listeners
-        mTray.removeListeners(mWindows);
-        mTray.onDestroy();
         mWindows.onDestroy();
 
         SessionStore.get().onDestroy();
@@ -1207,7 +1205,9 @@ public void popWorldBrightness(Object aKey) {
 
     @Override
     public void setTrayVisible(boolean visible) {
-        mTray.setTrayVisible(visible);
+        if (mTray != null && !mTray.isReleased()) {
+            mTray.setTrayVisible(visible);
+        }
     }
 
     @Override

app/src/common/shared/org/mozilla/vrbrowser/ui/widgets/TrayWidget.java

@@ -218,10 +218,6 @@ public void removeListeners(TrayListener... listeners) {
         mTrayListeners.removeAll(Arrays.asList(listeners));
     }
 
-    public void onDestroy() {
-        mTrayListeners.clear();
-    }
-
     private void notifyBookmarksClicked() {
         mTrayListeners.forEach(TrayListener::onBookmarksClicked);
     }
@@ -266,6 +262,7 @@ public void releaseWidget() {
         }
 
         mWidgetManager.removeUpdateListener(this);
+        mTrayListeners.clear();
 
         super.releaseWidget();
     }

app/src/common/shared/org/mozilla/vrbrowser/ui/widgets/UIWidget.java

@@ -49,6 +49,7 @@ public interface Delegate {
     protected int mBorderWidth;
     private Runnable mFirstDrawCallback;
     protected boolean mResizing = false;
+    protected boolean mReleased = false;
 
     public UIWidget(Context aContext) {
         super(aContext);
@@ -203,6 +204,11 @@ private void releaseRenderer() {
     public void releaseWidget() {
         releaseRenderer();
         mWidgetManager = null;
+        mReleased = true;
+    }
+
+    public boolean isReleased() {
+        return mReleased;
     }
 
     @Override