Flutter - cannot access listener in Native code

I am trying to make a plugin for an SDK in Flutter, I have written my entire code in Java however when I compile my file and my java/io file gets generated it shows me the error that it cannot access a following listener that I have added to my Java file -

error: cannot access TheoremReachRewardListener
    flutterEngine.getPlugins().add(new com.example.trsurveys.TrsurveysPlugin());
                              ^
  class file for theoremreach.com.theoremreach.TheoremReachRewardListener not found

I have tried adding the file manually as an AAR file in my build.gradle and when I compile this from their android example file everything compiles. Could this be a problem in the SDK itself or am I missing something in my native code?