Can't use "Dynamic Property/Method" in GSP file (Groovy Server Page) Follow

I am running IU 135.760 and I just tried to edit a Groovy script containing some dynamic properties.

Typically, when I alt+Enter on an unresolved identifier, IDEA suggests me to define it as dynamic property, so I can specify the type and get auto-completion, but for some reason it doesn't work now. I only get all the 'Create xxx' intentions...

Moreover, the side-panel listing the dynamic stuff is gone, which leads me to think that I have disabled functionality or something is very wrong.

Can anybody reproduce? Does anybody has any idea which plugin provides this functionality?