Calling a tab fragment method inside the parent acitivity

I want to simply call a fragment method inside my MainActivity.

Sound simple but in my case I can’t use the FragmentManager to call it because I have a TabView in my app, so I have to call the TabFragment which manages all the other tab Fragments first:

mFragmentManager = getSupportFragmentManager();
mFragmentTransaction = mFragmentManager.beginTransaction();
mFragmentTransaction.replace(R.id.containerView, new TabFragment()).commit(); 

Normaly I would simply use .commitNow() to call the fragments method afterwards.
But in my case I don’t want to call a method out of my TabFragment but out of a Fragment which gets called inside the TabFragment.

How can I achieve this?

I already tried an interface but got nullPointerExceptions all the time, is an interface the right way to do it?

Here is my previous question where I explained how I used the interface which did not work for me:
How to use interface to communicate between fragment and activity?

Thanks