Umutcan can you describe the steps to reproduce in more details, maybe with screenshots or a video? I don't understand which part of the software you are referring to.
Are you using the Flarum Package Manager which is in beta?
Or are you referring to the built-in feature called "Purge" (confusingly called "Uninstall" in old Flarum versions, maybe not yet renamed in every language pack), which deletes the data for an extension but doesn't actually remove it?
I'm not familiar with the Package Manager but any built-in Flarum feature should not result in a white screen. Do you see any error in the Flarum log file or in the browser javascript console?