The return type 'String' isn't a 'Future<void>', as required by the closure's context.

This error occurs when a function or closure is expected to return a `Future<void>` but instead returns a `String`.

To fix this error, you need to make sure that the function or closure returns a `Future<void>` instead of a `String`. You can do this by replacing the `return` statement with `return Future.value(null);` or simply `return null;`.

For example:

Future<void> myFunction() async {
  // do something
  return null;
}

Or

Future<void> myFunction() {
  // do something
  return Future.value(null);
}