Description of problem: See linked issue.
The Cancel event definition is only valid if the End event is contained in a Transaction SubProcess (see Table 10.88 of the BPMN 2.0 spec). The fact that you can add a Cancel event definition from the toolpalette is a bug.
Verified on BPMN2 Editor 0.2.7.201309191900 (JBTIS 4.1.3.Beta3)
This BZ has been part of the 6.0.0 stream.