Yeah, I just figured out that the former developer has overwritten the built-in "text" plugin.
This is exactly the type of thing that creates horrible compatibility nightmares. Developers should never do that and users should never use any addons that do that. You are going to have a really tough time because of this.
Your best bet is to try to remove all references to the broken plugin (as doing this is just a way to break everything), then rebuild the missing parts in the editor once you get it to import.