Fix Theme overriding existing theme types with empty

[YuriSizov]

These methods are only used in the theme editor, and are not exposed to the scripting API.

To reproduce the issue:

1. Create an empty Theme resource.
2. Click on "Manage Items" and add a new type.
3. Create an arbitrary theme item for that type.
4. Add the same type once again — added item will be deleted.

[akien-mga]

Thanks!