In my case I had a library module with an abstract application class implementing Dagger’s HasActivityInjector and an application module with a dependency to the library and its own (non-abstract) application class extending the base app class.
My Gradle dependencies were implementations, and therefore not accessible to the application module’s application class (even though there were no imports there, otherwise the issue would be immediately obvious as you’d get a ‘could not resolve’ error. The fix was to replace dagger implementation dependencies with api, that makes them available to dependant modules as well.