Can't use baseLanguage.extensionMethods inside typesystem aspect (unengaged in generation plan)

I call methods from the behavior aspect inside checking rules of the typesystem aspect. So far this was ok, but now I started calling an extension method that I wrote. When I try to make the typesystem aspect now, it gives me this error:

Model uses language jetbrains.mps.baseLanguage.extensionMethods with 1 generator(s), none of which is engaged with the active generation plan

When click on ignore errors, it generates this into the java file and therefore fails:

<!TextGen not found for 'jetbrains.mps.baseLanguage.extensionMethods.structure.ExtensionMethodCall'!>

When I add it to the engaged languages, it warns me that it's unnecessary:

When I then try to make the model now, it does not warn me about a model checker error anymore, but still generates the Java file incorrectly as before. I don't know what all of this means, I never wrote a generation plan explicitly. Apparently this is about some internal MPS plan. Is it just impossible to call extension methods from the typesystem aspect?