Cannot preview a Theme with a bespoke Grammar

[cabintech]

On the Theme page of the TextMate preferences the drop-list of grammars does not include all defined grammars. It is not clear what makes a grammar eligible for this list, but my custom grammar (loaded from a .tmLanguage file, not a plugin) is not in the list so I cannot preview my custom theme with my custom grammar.

[sebthom]

The themes preview drop-down only contains grammars for which a preview text is registered. So if a preview text could be registered for manually configured grammars (#838) this issue would be resolved.