Resolve IllegalStateException, Fragment.requireContext

Even if isAdded is called just before showConnectingDialog, it may be
disconnected once we get in the main thread after getting the
preferences.

Squeezer/src/main/java/uk/org/ngo/squeezer/NowPlayingFragment.java

@@ -223,6 +223,11 @@ public void onReceive(Context context, Intent intent) {
     private void showConnectingDialog() {
         if (connectingDialog == null || !connectingDialog.isShowing()) {
             Squeezer.getPreferences(preferences -> {
+                // We may no longer be attached to the parent activity. If so, do nothing.
+                if (isAdded()) {
+                    return;
+                }
+
                 Preferences.ServerAddress serverAddress = preferences.getServerAddress();
 
                 final View view = LayoutInflater.from(mActivity).inflate(R.layout.connecting, null);