Error in getting result from Checkbox in ExpandableListView

I am new and very beginner to Android development. I am trying to get the textView from ChildList (expandableListView) where checkbox is selected but I am getting null reference error on running the code. Can you please help. Below is the error

Attempt to invoke virtual method 'java.lang.String java.util.ArrayList.toString()' on a null object reference

below is the Extract of code.

I am calling this in the main activity to get the arraylist of text for which checkbox is selected.

public String getSelectedItems(int mGroupPosition)
{
    boolean getChecked[] = mChildCheckStates.get(mGroupPosition);
    ArrayList<String> selectedList = null;
    if(getChecked != null) {
        for (int j = 0; j < getChecked.length; ++j) {
            if (getChecked[j] == true) {
                String selectedItem =  mListDataChild.get(j).toString();
                selectedList.add(selectedItem);
            }
        }
    }
    return selectedList.toString();
}

Code in Main Activity

selectedButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ArrayList<String> selectedItemsList = null;
                for (int mGroupPosition = 0; mGroupPosition<listAdapter.getGroupCount(); mGroupPosition++){
                    selectedItemsList.add(listAdapter.getSelectedItems(mGroupPosition));
                }
                Log.i("selected Items",selectedItemsList.toString());
            }
        });