Description of problem:
After planing a remote command it's not possible to cancel the planned event from the pending event tab of the system but canceling the event from the "Schedule" tab works.

Version-Release number of selected component (if applicable):
1.6

How reproducible:
Go to desired system, plan an action (e.g. run remote script). Go to events -> pending, select event, cancel and see error 500.

Steps to Reproduce:
1. select system
2. schedule remote command
3. go to events -> pending
4. select event
5. cancel event
  
Actual results:
Internal Server Error 500

Expected results:
Cancel event.

Additional info:
Canceling desired events from "Schedule -> Pending Actions" tab works just fine