NullPointerException when Android fragment onCallBack with data

Question

I am trying to pass data between fragments on call back. In my first fragment:

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View v = ...

    categoryList = (ArrayList<Category>) getArguments().getSerializable("categorylist");
    return v;
}

@Override
public void callbackCall(ArrayList<Category> list)
{
    Toast.makeText(getActivity(), "call back",
            Toast.LENGTH_SHORT).show();
}

My interface:

public interface CategoryListCallBack {
void callbackCall(ArrayList<Category> list);
}

In my second fragment when I successfully updated the database, I am pulling the data again to be pass back to the first fragment:

private CategoryListCallBack mcallback;

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = ...

        mcallback = (CategoryListCallBack) getFragmentManager().findFragmentByTag("CategoryListActivity");

return v;
}

final CategoryListActivity categoryFragment = new CategoryListActivity();
    new GetAllCategoriesAsyncTask(
            new GetAllCategoriesAsyncTask.OnRoutineFinished() {
                public void onFinish() {
                    getFragmentManager().popBackStack();

                    /*Bundle bundle = new Bundle();
                    FragmentTransaction it = getFragmentManager().beginTransaction();

                    bundle.putSerializable("categorylist", GetAllCategoriesAsyncTask.categoryList);
                    categoryFragment.setArguments(bundle);
                    it.replace(R.id.frame,categoryFragment);
                    it.addToBackStack("tag");
                    it.commit();*/
                    mcallback.callbackCall(GetAllCategoriesAsyncTask.categoryList);
                }
            }).execute();

However, I am getting this error message:

java.lang.NullPointerException: Attempt to invoke interface method 'void Controller.CategoryListCallBack.callbackCall(java.util.ArrayList)' on a null object reference

Any ideas? Thanks in advance.

Edit

When my list view item onclick, I am calling the second fragment:

Bundle bundle = new Bundle();
            EditCategoryActivity editFragment = new EditCategoryActivity();
            FragmentTransaction et = getFragmentManager().beginTransaction();

            bundle.putSerializable("category", model);
            editFragment.setArguments(bundle);
            et.replace(R.id.frame,editFragment);
            et.addToBackStack("tag");
            et.commit();

In my second fragment onCreate():

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.activity_edit_category, container, false);

    mcallback = (CategoryListCallBack) getFragmentManager().findFragmentByTag("CategoryListActivity");
    //v.setCategoryListCallBack(mcallback);

    // get category details pass from previous intent
    category = (Category) getArguments().getSerializable("category");

    // button onclick insert data to database, then call the async task and call back to first fragment
    return v;
 }

Answer 1

In your second fragment

mcallback = (CategoryListCallBack) getFragmentManager().findFragmentByTag("CategoryListActivity");

this line must be returning null ,try this code

public void setCategoryListCallBack(CategoryListCallBack categoryListCallBack){
    this.mcallback = categoryListCallBack;
}

set interface reference from this method ,and always do null check on interface reference

final CategoryListActivity categoryFragment = new CategoryListActivity();
new GetAllCategoriesAsyncTask(
    new GetAllCategoriesAsyncTask.OnRoutineFinished() {
    public void onFinish() {
        if(mcallback != null){
            mcallback.callbackCall(getContext());
        }
        getFragmentManager().popBackStack();

            /*Bundle bundle = new Bundle();
            FragmentTransaction it = getFragmentManager().beginTransaction();

            bundle.putSerializable("categorylist", GetAllCategoriesAsyncTask.categoryList);
            categoryFragment.setArguments(bundle);
            it.replace(R.id.frame,categoryFragment);
            it.addToBackStack("tag");
            it.commit();*/

    }
}).execute();

Add listener for second fragment like:

Bundle bundle = new Bundle();
EditCategoryActivity editFragment = new EditCategoryActivity();
FragmentTransaction et = getFragmentManager().beginTransaction();
editFragment.setCategoryListCallBack(this);
        bundle.putSerializable("category", model);
        editFragment.setArguments(bundle);
        et.replace(R.id.frame,editFragment);
        et.addToBackStack("tag");
        et.commit();

and remove this line from onCreatView()

mcallback = (CategoryListCallBack) getFragmentManager().findFragmentByTag("CategoryListActivity");