You have 2 sections in kstars configure when you control the fits preview :
- in fits you could change the preview "use fits viewer"
- in ekos, on tab capture, you could check/uncheck "summary screen preview"
I don't know exactly than you must uncheck, but i think than one of 2 do what you want.
There's also a fits viewer toggle button you can add to the tool-bars. I found that flipping it can stop this behavior.
I've also seen it crash KStars, don't know why.