I just encountered this error shown below and pressed the Report Error button to send it off. As you can see there are 3 appointments shown - I clicked the bottom one.

If I go and look at the schedule, the appointment is there and I can edit it.  HOWEVER, note that in the above snippet, the practice location is set to ZZ Test (our play/test location) - which happens to have no associated schedule or schedule view.  If on the above screen, I change to a 'real' location (with schedules) then it works as expected.

Recommended Action: don't spend a lot time trying to fix the bug- this is funny setup. I will tweak ZZ Test so it has a schedule view and thus avoid the problem.