code: private InvocationHandlerAdapter getInvocationHandlerAdapter(Object marker) {
          if (marker == null) {
              return null;
          }
  
          return markerToHandler.get(marker);
      }
docs: Get the {@link InvocationHandlerAdapter} registered for a marker.
  
  @param marker marker of the class that might have mocking set up
  @return adapter for this class, or {@code null} if not mocked