Eclipse Display view does not work while debugging Groovy.
How can I get a REPL when debugging Groovy?
Edit:
It is worth noting the problem is not related to the given code. A simple expression like 1+1 also has this problem. So I assume Groovy doesn't work with Display.
This will not work with vanilla groovy-eclipse. The only way to get display view, watch expressions, and conditional breakpoints working with groovy is if you install GGTS. The reason is that the implementation requires additional hooks into the Java compiler and tools not available without GGTS.
The display view integration is not perfect, but it will work for most cases in groovy (including being meta-programming aware).
Just make sure that JDT weaving is enabled (go to Preferences -> JDT weaving to configure).