My recyclerview is NULL even though I am passing a valid one

Question

I found a bug in my code, where the recyclerview may be null. How can I fix this?

These are the classes and code:

This is my UserListRecycler class - the main class

public class UserListRecycler extends Fragment {
    RecyclerView recyclerView;
    static UserAdapter adapter;
    RecyclerView.LayoutManager layoutManager;
    ArrayList<UserInfo> list;


    @Override
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
    }

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.userlistGUI, container, false);
        recyclerView = (RecyclerView) rootView.findViewById(R.id.reUsers);
        recyclerView.setHasFixedSize(true);
        list = new ArrayList<UserInfo>();
        // Instantiate new adapter here
        adapter = new MusicRecyclerAdapter(list);
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
        linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
        recyclerView.setLayoutManager(linearLayoutManager);
        // Sets the adapter here
        recyclerView.setAdapter(adapter);
        adapter.notifyDataSetChanged();
        return rootView;
    }

    @Override
    public void onStart() {
        super.onStart();
        populateRecyclerList();
    }


    public void populateList(){
        PopulateUsers userList = new PopulateUsers(list, adapter, recyclerView);
        userList.execute();
    }
}

This is my PopulateUsers class - where details from my server are retrieve via REST

public class PopulateUsers extends AsyncTask<String, String, String> {
    static ArrayList<UserInfo> list;
    UserAdapter adapter;
    RecyclerView recyclerView;

public PopulateUsers(ArrayList<UserInfo> list, UserAdapter adapter, RecyclerView recyclerView) {
    this.list = new ArrayList<UserInfo>(); 
    this.adapter = new UserAdapter();
    this.list = list;
    this.adapter = adapter;
    this.recyclerView = recyclerView;
}

@Override
protected void onPreExecute() {
    super.onPreExecute();
}

@Override
protected String doInBackground(String... params) {
    // `REST` Activity happens here
}

@Override
protected void onPostExecute(String s) {
    try {
        // IT CRASHES HERE, BUT IF I INSTANTIATE A NEW LIST, 
        // THEN THIS ERROR GOES AND NOTHING IS DISPLAYED
        list.clear();  
        JSONArray jsonArray = new JSONArray(s);
        for (int i = 0; i < jsonArray.length(); i++) {
            String forename = jsonArray.getJSONObject(i).getString("forename");
            String surname = jsonArray.getJSONObject(i).getString("surname");
            String nationality = jsonArray.getJSONObject(i).getString("nationality");
            UserInfo userInfo = new UserInfo(forename, surname, nationality);
            list.add(userInfo);
        }
        // THIS LINE BELOW IS THE BIGGEST CULPRIT. IT HAS BEEN A HUGE HEADACHE
        // I EVEN TRIED THIS: adapter = new MusicRecyclerAdapter(list);
        // AND IT STILL CRASHES
        recyclerView.setAdapter(adapter);
        adapter.notifyDataSetChanged();
    } catch (JSONException e) {
        e.printStackTrace();
    }
}

}

The adapter is NULL but how can it be friggin NULL if I have initialised it!

It keeps crashing when I call the populateList() method in another AsyncTask class - see below:

public class RecommendUsers extends AsyncTask<String, String, String> {
    @Override
    protected String doInBackground(String... params) {
        // REST stuff happens here
    }

    @Override
    protected void onPostExecute(String postData) {
        // The following line invokes the instance of `PopulateUserList` and crashes.
        // This is the culprit line
        populateList();
    }
}

This is the stack trace:

 java.lang.NullPointerException
            at lukasz.usersapp.PopulateUsers.onPostExecute(PopulateUsers.java:67)
            at lukasz.usersapp.PopulateUsers.onPostExecute(PopulateUsers.java:27)
            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)

Basically it crashes at list.clear(), but that is not an issue - I can fix, but the follow up - the recyclerView.setAdapter(adapter) is the one that is giving me a big headache. Literally driving me bonkers!!!

Would appreciate some support or solutions

In the PopulateUsers constructor, I am initialising, but I feel that the recyclerview may be NULL hence my code crashing - please see the constructor below:

public PopulateUsers(ArrayList<UserInfo> list, UserAdapter adapter, RecyclerView recyclerView) {
    this.list = new ArrayList<UserInfo>(); 
    this.adapter = new UserAdapter();
    this.list = list;
    this.adapter = adapter;
    this.recyclerView = recyclerView;
}