Fix actor-isolated class is different from nonisolated subclass error

I'm trying to migrate my fairly large application to Swift Concurrency. I've have a class marked as @MainActor that sub-classes a 3rd party abstract class that is not migrated to Swift Concurrency.

I get the following error:

Main actor-isolated class 'MyClass' has different actor isolation from nonisolated superclass 'OtherAbstractClass'; this is an error in the Swift 6 language mode

My class needs to be MainActor as it uses other code that is required to be on the MainActor.

I can't see how to suppress this warning, I know as a guarantee that the abstract class will always be on the main thread so I need a way of telling the compiler that when I don't own the 3rd party code.

import  OtherAbstractModule

@MainActor
class MyClass: OtherAbstractClass {
    ....
}

How can I satisfy the compiler in this case?