And I AM a little blind! While creating intents and thinking about the right context (getActivity(), getParentFragment().getActivity(), ...) or other possibilities to create callbacks to my fragment I DID NOT waste a second thinking about some other reasons for the error. So the EditText-Element I try to get the text from (mEditText) was not initialised :\
Some modification to make it work:
View v = inflater.inflate(R.layout.dialog_choose_title, null);
mEditText = (EditText) v.findViewById(R.id.id_edit_choose_title);
mEditText.setText(mTitle);
builder.setView(v);
Thanks for your time Gokhan!