Ok, this is my solution (maybe not the best way, but it's working ):
In Choose.java, the String storage is declared with
If I replace Default with my path to the file and the app is started and you directly go to Main and press the button this String is loaded.
If you open Settings first the text from the EditText is taken.
This is how storage is looking now:
I just have to read into the ExternelStorage-thing to make it work on each device, but that shouldn't be that problem (I hope).