I ran across this thread on the Forge forums last night. Evidently SK's launcher is not a "proper launcher", whatever that means Here's the full quote: Definitely not feeling the love. As far as I can tell, the error is caused by incorrectly copying forge into the minecraft jar, and does not have anything to do with the launcher.
I think that's an old post from when the launcher didn't support applet replacement, which crashed Forge's seperate useless setup window.