Ecliipse Windows Builder Problems

I hace a problem with eclipse and windows builder. When i change the tab from "Source" to "Design" for some reason windows builder (I think its windows builder, im not sure) open me another JFrame even in other package. It doesnt matter what class i have open, sometimes, just when I change from source tab to design tab it opens the JFrame(always the same JFrame) and crash eclipse. I tried rename it and still the same. Does someone know how to solve it?

My eclipse version.

Version: Luna Service Release 2 (4.4.2)
Build id: 20150219-0600