ExpandableListView null object Reference

Question

I have an ExpandableListView in my Fragment that shows some data. I call the Fragment when i press an icon.

public void searchClick(){
        Fragment frag = new Results();

        FragmentTransaction ft  = getSupportFragmentManager().beginTransaction();

        ft.replace(R.id.FragFaecher,frag).commit();



        displayList();


    }

displayList() then should populate the List and show it

public void displayList(){
        populateList();

        expList = (ExpandableListView) findViewById(R.id.expList);
        listAdapter = new ExpListAdapter(MainActivity.this,parentList);
        expList.setAdapter(listAdapter);

    }

In FragmentClass the Layout with the ExpandableListView is inflated within the fragment_results Layout

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


        return rootView;
    } 

fragment layout:

  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent" android:layout_height="match_parent"
        android:id="@+id/results"
        android:paddingBottom="@dimen/activity_vertical_margin"
        android:paddingLeft="@dimen/activity_horizontal_margin"
        android:paddingRight="@dimen/activity_horizontal_margin"
        android:paddingTop="@dimen/activity_vertical_margin"
        android:background="?android:colorBackground"
        android:clickable="true">


       <ExpandableListView
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:background="?android:colorBackground"
            android:childDivider="#000000"
            android:id="@+id/expList" />


    </RelativeLayout>

I suppose the problem is, that the displayList() method tries to find a View that is not yet created. I tried to return a boolean when the View was created, but that did not work. Is there any other possible way to prevent this from happening?