Issues #10914: Prevent crash due to non-existent editors
For @lillolollo's testing purposes.
Some dockables such as "gimp-pattern-editor" and "gimp-mybrush-editor" are listed as Editors even though we don't yet have dedicated GimpDataEditors for them.
This causes problems when using certain features like duplicating. To resolve for now, we check if gimp_window_strategy_show_dockable_dialog () returns a valid GimpDataEditor before trying to use it.