android - can't access view inside fragment method. it returns null object

I'm building an app based on the tabbedActivity preset of android studio ,therefore fragments are created so:

private SectionsPagerAdapter mSectionsPagerAdapter;

    private ViewPager mViewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);
}

and later on:

public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            switch (position) {
                case 0:
                    DashboardTab tabDashboard = new DashboardTab();
                    return tabDashboard;
                case 1:
                    VertretungsplanTab tabVertretungsplan = new VertretungsplanTab();
                    return tabVertretungsplan;
                default:
                    return null;
            }
        }

        @Override
        public int getCount() {
            // Show 2 total pages.
            return 2;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
                case 0:
                    return "Dashboard";
                case 1:
                    return "Vertretungsplan";
            }
            return null;
        }
    }

this is all working fine but in fact it is quite an uncommon way of creating tabs so it might be that my problem is caused by a missunderstanding of the MainActivity.java

public class VertretungsplanTab extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.vertretungsplan_fragment, container, false);
        return rootView;
    }



    public void setText(String textToBeSet, VertretungsplanTab tab){
        TextView tv = (TextView) getView().findViewById(R.id.textView2);
    }

now the problem is as far as i now caused by the view, because it is a null object inside my method.

Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference

I've tried several ways of accessing the view, which includes creating a new variable in this fragment and commiting the view to it in onCreateView(). Also if I checked it in onCreatedView() it wasn't yet a null object.

I've also tried to create the TextView in onCreateView(), but after onCreatedView() it isn't possible to call setText()

After searching the internet several days it would be a great help if a more advanced developer could get a solution!