SourceKitService Crashed

This is a bug in Xcode and you cannot do much about it.

Update to Xcode 6.2 Beta 1 or higher if you want as the situation improved there. Still happening occasionally though.

Have this problem with Xcode 6.1 (release, not GM).

There a few "magic" solutions that works temporarily, but require a restart. There is quick fix that seems to hold (and does not require a complete restart).

Delete the content of: DerivedData/ModuleCache

(Full path: ~/Library/Developer/Xcode/DerivedData/ModuleCache)

Try emptying the ~/Library/Developer/Xcode/DerivedData folder. That should fix the problem.