Xcode 10.2.1 can't Index or Build any project

Question

Seemingly out of nowhere, my Xcode (10.2.1) became unable to build or even index any single project I try to Run, even an unchanged "Single View App" preset. And my Xcode 11.0 Beta 4 crashes any time I try to open a file inside of a project. What should I check and/or try doing?

macOS Catalina (10.15 Beta)

Answer 1

Just restart your computer, that is what helped in my case