A class contains a nested class definition, and the nested class is not marked with the static modifier. Another defined class extends the nested class, but the extending class either does not have the correct modifiers or is not defined within the same class as the nested class. Only another non-static class that is nested within the same class can extend a non-static nested class.
To correct this error
Make sure that only non-static nested classes extend non-static classes that are nested within the same class.
Apply the static modifier to the nested class that is to be extended. This allows non-nested classes and static nested classes to extend the nested class.