How do I call an outer class method in my inner cl

So I was having a bug where my code was crashing because I am calling methods in my outer class in onPostExecute.

So this is my simple class:

public class UsersList extends Fragment{
 @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        ...
    }

    public void callMe(){
        Toast.makeText(getContext(), "I work", Toast.LENGTH_LONG).show();
    }


    public class UserRecommend extends AsyncTask<String, String, String> {

            @Override
            protected String doInBackground(String... params) {
                ...
            }

            @Override
            protected void onPostExecute(String data) {
                // The line below crashes with NullPointerException
                UsersList.this.callMe();
            }
        }
}

So UserRecommend is an inner class in UsersList, and so I am calling the callMe() method and it is crashing. It is crashing for any method in my outer class which is UsersList. I know I can print toast directly, but I don't want to as I have a more complex method. How do I fix this, I need to call a method in the outer class

EDIT: I found the main bug - but I don't know how to fix it. Basically, I have a custom recyclerview adapter and that cardview has a recommend button. When the user clicks the recommend button - it invokes a method which handles the db stuff and recalls a populate method. I feel that this is where the problem lies but here is the bit where I define the button action:

 @Override
    public void onBindViewHolder(final RecyclerViewHolder holder, int position) {
        holder.recommendButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                UsersList usersList = new UsersList();
                UsersList.UsersRecommend usersRecommend = usersList.new UsersRecommend(usersList);
                usersRecommend.execute();
            }
        });
    }

That above method is in my CustomAdapter class and that button is defined in that class and not my main UsersList class

Stack trace:

java.lang.NullPointerException
        at com.android.volley.toolbox.Volley.newRequestQueue(Volley.java:43)
        at com.android.volley.toolbox.Volley.newRequestQueue(Volley.java:78)
        at lukazs.usersapp.UsersList.getRView(UsersList.java:186)
        at lukazs.usersapp.UsersList.callMe(UsersList.java:161)
        at lukazs.usersapp.UsersList$UserRecommend.onPostExecute(UsersList.java:459)
        at lukazs.usersapp.UsersList$UserRecommend.onPostExecute(UsersList.java:431)
        at android.os.AsyncTask.finish(AsyncTask.java:631)
        at android.os.AsyncTask.access$600(AsyncTask.java:177)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5103)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:525)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
        at dalvik.system.NativeStart.main(Native Method)

Because I am calling the UserRecommend class from another CustomAdapter class, I believe it is losing the activity. It is crashing on this specific line: RequestQueue requestQueue = Volley.newRequestQueue(getActivity());

This is my code for where I call this function:

 @Override
    public void onBindViewHolder(final RecyclerViewHolder holder, int position) {
        holder.recommendButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                UsersList usersList = new UsersList();
                UsersList.UsersRecommend usersRecommend = usersList.new UsersRecommend(usersList);
                usersRecommend.execute();
            }
        });
    }