I wasn't entirely sure where to post this, and I saw this thread already, and this is my exact problem I just encountered.
I downloaded Stencyl a few days ago and have been fooling around with it, trying to teach myself the program and figure out how to create and edit behaviors using the design mode. Needless to say it's been fun. Until, I too just got this same error.
The game is nothing that completely formed yet. I was just using Mario sprites to teach myself this program, but I still had some of my own behaviors I was working on, and trying to figure out. Any ideas what made this become corrupt?