[Refactoring-UI] Error when renaming method to existing selector

[JanBliznicenko]

When we try to rename a method a name that already exists in a class, the reactoring offers to do it or show the violators. Instead of showing them, it ends up with an error.

Steps to reproduce:

1. Create a method named 'abc' in any class
2. Create a method named 'def' in the same class
3. Right click the 'def' method
4. (R) Rename
5. Fill name 'abc' (the other method)
6. Pick "Browse existing methods that will be overriden BUT don't rename" and Accept

=> Instance of Array did not understand #violators, in ReRenameMethodRefactoring

Issue is present in the released Pharo 13 and current build of Pharo 14.