Close button sends client to the tray, statically linked Qt for OSX.
See the Changelog for list of features and fixes currently only available in the nightly builds.
If you're on a Mac, bear in mind that SoulseekQt requires Snow Leopard or newer to run, and will crash if you're using an older operating system. Otherwise, the best thing you can do to help fix the cause of the crash is to post a crash report. See this article for instructions.