I notice two keys that might be relevant: NSRecentXCFileDocuments and NSRecentXCProjectDocuments. Both appear to be arrays, so you could reset one like this:
I just spent half the day stressing over a very similar issue. I had tried updating and reinstalling Xcode - but the problem still persisted. Thankfully, a few minutes ago, I managed to solve this by doing what your question states with the help of 这篇文章.
基本上,我使用以下命令从终端打开了另一个 Xcode 项目(尽管它可以是任何文件) :
open -a /Applications/Xcode.app app.xcodeproj/ --args -ApplePersistenceIgnoreState YES
Press option+Shift when clicking on the Xcode icon in the dock. Xcode then starts without opening previously used projects. This is related to version Version 4.5.2 (4G2008a) but I am almost sure that I used it in older versions as well.