Crash was happening in this scenario:
- mouse was over a property in the dialog
- operator had a "check" callback that was accessing certain operator
stuff
In this case, the `UI_BUT_ACTIVE_DEFAULT` button was executed, operator
finished or canceled (but stuff was freed along the way), then code
kicked in to still handle ENTER over the property in the popup (e.g.
toggle a checkbox), the `uiAfterFunc` still had the `popup_op`, tried to
run `popup_check` again with the faulty data.
Simple way to fix this is to use `WM_UI_HANDLER_BREAK` to not handle
further buttons after executing the `UI_BUT_ACTIVE_DEFAULT`
