Found the log file. Here's the relevant part:
info startup: Initializing user interface rendering...
error window: Application creation error: java.lang.RuntimeException: asset not loaded: shader/gui.json
at com.lezliz.b.g.a(Unknown Source)
at com.lezliz.rainnskye.b.e(Unknown Source)
at com.lezliz.rainnskye.a.a(Unknown Source)
at com.lezliz.window.MainWindowNative.a(Unknown Source)
at com.lezliz.rainnskye.Main.main(Unknown Source)