Fix NullPointerException on runOnMainThread

Fix the issue #190 using the same native Android SDK implementation.

Importing libraries Handler and Looper

Importing libraries

Update FlutterRegistrarResponder.java

Author: rafael-matsuda

android/src/main/java/com/onesignal/flutter/FlutterRegistrarResponder.java

@@ -1,5 +1,8 @@
 package com.onesignal.flutter;
 
+import android.os.Handler;
+import android.os.Looper;
+
 import android.app.Activity;
 
 import java.util.HashMap;
@@ -54,7 +57,12 @@ public void run() {
    }
 
    private void runOnMainThread(final Runnable runnable) {
-      ((Activity) flutterRegistrar.activeContext()).runOnUiThread(runnable);
+      if (Looper.getMainLooper().getThread() == Thread.currentThread())
+         runnable.run();
+      else {
+         Handler handler = new Handler(Looper.getMainLooper());
+         handler.post(runnable);
+      }
    }
 
    void invokeMethodOnUiThread(final String methodName, final HashMap map) {