If you use an older version of WorkflowGen.My 2.1.5, then the error is: Object reference not set to an instance of an object.
If you use the version 2.1.5 of WorkflowGen.My or later, then the error is Session Timeout.
To avoid this error message, increase your session timeout in IIS for your web form, or do not use the session object in your webform by adding the following line in the constructor of your web form:
This.IsSessionLess = true;