I seem to be getting a similar error to this one and also this one: https://www.genuitec.com/forums/topic/error-when-eclipse-go-up-swtexception/
I got this on an old myeclipse 5.0-ish workspace that I upgraded to myEclipse 6. I also get this on a new computer with a fresh workspace. A coworker is having the same problems. It seems to be tied to restarting tomcat, but it only happens about 1 time in 10. Once the error starts happening I have to restart myEclipse to get it to quit happening.
FYI, -clean does not help.