Hello,
Can you check once what is the return type for ‘workflowProperties.Item["myCategory"]'? If return type is node then you have change your code as below,
workflowProperties.Item["myCategory"].value = _AfterProperties1.ExtendedProperties["myCategory"].ToString();
or
workflowProperties.Item["myCategory"].text = _AfterProperties1.ExtendedProperties["myCategory"].ToString();