This is a very subtle bug which has to do with the Observing Lists
I found it on multiple occasions over a period of several days on two computers.
What one should do to find this bug is :
Select an observing list
Navigate through the list with the Up and Down keys
Highlight the objects of the list and Select Next Object
Click on an object of the chart which doesn’t belong to the highlighted list
Return to the Observing List and select an object
At one of these stages, the popup Show List at the bottom of the chart will not respond anymore and the only way to get out is a Force Quit.
SkySafari 6 Pro on macOS High Sierra