Update regarding this topic. Everything works now.
What I did was:
First I deleted the "forms" folder in the folder WebAssist. Then I created a new form from the preset that I saved.
I do not know if this is classed as a bug. But I think it's a bit strange that you can not work on the form it once created. Maybe it's because I've gone into the form and manually added the code, or that I have created many different versions of the form so it will be too much for the Form Builder to stick together, or to write the correct files.