@jom & @Angelych:
I will start my modification on navboot and save it prompt with the name "1".
then after the 1. change i´ll save it as "2".
after every each change I save the file immediately. The name to store will change between "1" and "2". So "1",next again "2", next "1" ....
So I have still the last change working, before navtweak will crash.
At last I close navtweak and start it again to load the last change file. Then save it as navboot. When it won´t crash, so i will use it fpr my images.