backend destroyed when plugin is deactivated

Hi,
I tried to test several plugins for syntax-highlighting, but no plugin was working. I thought, maybe it was related to the classic editor and tried to deinstall it. But then I realized that the backend is being destroyed, when I do this, so there is nearly only a blank screen! The frontend of my site is ok! I deleted the cache, but it didn’t work.
Has anyone an idea, why I can’t deinstall the classic editor?

Thank you very much in advance!
Andreas