The logs show a different error:
2015-02-25 13:19:53,375 DEBUG [AWT-EventQueue-0] stencyl.sw.util.FileHelper: Deleting: C:\Users\Ярослав\AppData\Roaming\Stencyl\stencylworks\preview\game
2015-02-25 13:19:53,414 ERROR [pool-1-thread-1] stencyl.core.lib.Game: unknown protocol: c
java.net.MalformedURLException: unknown protocol: c
This one was caused by the non-English chars in your username. It is fixed already in the private build / upcoming version.