Thank you for your enquiry.
This is indeed a known issue, which is addressed by the following (quoted from the Formativ Readme document):
– Not closing certain item editors in the Form Designer prior to saving the form may cause an access violation. You should manually close any popup form editors prior to saving a form.
At this stage this is the only workaround that I know of. So for example, after using the ListView columns editor, you must close the editor before you close the Forms Designer (whether you save or cancel changes).
I hope this information is helpful.