Crash if only one satellite is selected and the user wants to do satellite planning

[komacke]

Describe the bug
If only one satellite is selected and then Satellite planning is selected, HamClock crashes

To Reproduce
Steps to reproduce the behavior:

1. select a single satellite, for example ISS
2. go into the satellite pane
3. select "Show Planning Tool"
4. select "Ok"
5. HamClock crashes with an exit/restart selection

Expected behavior
It shouldn't crash.

Screenshots