I want to prevent SourceKitService from crashing.

Just to be sure, are you using Xcode 6.1.1?:

The fix in 6.1.1 states that "many common causes of SourceKit crashes have been fixed": https://developer.apple.com/library/ios/releasenotes/DeveloperTools/RN-Xcode/Chapters/xc6_release_notes.html#//apple_ref/doc/uid/TP40001051-CH4-DontLinkElementID_10

Personally, I feel that not only 6.1.1 but also the latest version has improved, but if the problem continues at Masakih's place, only Apple can fix it, so I think I have to send you a bug report steadily: http://devforums.apple.com/