Thanks for the report. I can't reproduce this locally, but the likely candidate seems to be a bad ARC conversion in the class at the top of the stack trace. I'll double-check it and see if I can find the defect.
So, a little closer. If I start Synergy Prefs 6.0a3 with Synergy closed, it’s opens fine and I can do whatever in the app. Synergy controls iTunes fine. If I try to open it with Synergy running however, the Pref app crashes. The menu applet still works fine; the floater comes up and everything.