You may well be right, I haven’t tried adding a module at the engine level before. I looked at that function and it actually does very little, I can’t see how that would generate the error you’re seeing given that from your Bullet module you’re apparently not referencing any other engine modules.
There’s a question of what you mean by adding it to the engine. Do you intend to modify engine modules to use Bullet, or you just want the module to be available to projects? If it’s just the latter, then it would make more sense to create it as a plugin. Adding it as a dependency for the Engine module just to force it to be built when the Engine module doesn’t actually reference any of its symbols, doesn’t make much sense.
Other than that, I’m afraid I’m out of suggestions.