>>Check again, do you have any custom form references with Type="O"?
>
>Is there any convenient way to do this? All my controls are derived from base controls (Form -> GenericForm -> Form_with_grid etc.).

It might be some tool in Files section (I am not sure). Personally, I have a habit: when I create new property (Class/Form) with idea to use it for object reference, I immediately open Class/Form.Destroy event and set this property to .NULL.