I've just, unwisely so it would seem, got the latest IDEA EAP (637) and the latest Grails (2.3.1). When I loaded a project that I had set up in IDEA 12 and done a few things with it in both 12 and 13 EAPs - up to 556 - (and upgraded to Grails 2.3.0 a few weeks ago) and went to run Grails upgrade, IDEA failed. It auto-reported the failure which seemed to occur in two situations: immediately after the Grails upgrade reported 'project upgraded' and when I tried to set the JDK - the previous crash must have unset the JDK so that when I re-ran IDEA and opened the project IDEA reported 'JDK not set'.
What I then did was to go back to IDEA 12.1.4 and ran that and let it upgrade to 12.1.6. I was then able to run Grails upgrade completely successfully on the project (two in fact). And I could then return to EAP build 637 and run the application - at least to get the Grails fron screen and run a few trivial tests.
I can't see a way to find the report reference number (I didn't make a note of it...) but the error is shown as
Already disposed: URL=''
So far I haven't done very much with this version - only got both upgrades today.