Android : NullPointerException on calling Activity from FragmentActivity

I have an activity that extends FragmentActivity say MainActivity and it contained two Fragments say F1, F2. In F1 fragment, calling AsyncTask and implemented interface in MainActivity i.e., FragmentActivity. In that interface method, trying to call new activity say SecondActivity that extends Activity using below code but it throws exception.

Intent intent = new Intent(MainActivity.this, SecondACtivity.class);
startActivity(intent);

Please help.