Scenario:
Import a MOF file which contains super class reference from CIM schemas to a target model. During the Import, do not specify the reference model. Import should fail but Modeler didn't clean up the taget model properly. Therefore if you Import again with the same target model name, you will get an error even if you specify the reference model.
clean up code is added in the exception clause. When UMLBuffer fails, it exits with an exception.