Crash when inserting standard sample "Core shell particles"

Steps to reproduce:

1. Start BornAgain GUI
2. go to Welcome page, press "New project"
3. go to Sample page
4. in left sample list ("Samples"), select Library -> "Core shell particles"
5. -> CRASH

Going back in git, this shows that the error was introduced (or at least brought to visibility) with merge commit a8a09483 ("merge branch ff98 into develop").

The only commit in branch ff98 is "IBornFF now inherits from ISampleNode, no longer from IFormFactor"

With the previous merge commit ("merge branch ff97 into develop") this crash doesn't happen.

@j.wuttke You made this merge, could you please take a look? Maybe you see directly what's wrong.

The stack trace when crashing: