This bug was originally marked as duplicate of bug 201617, that is already resolved. This bug is still valid, so this seems to be another bug, but it might be related.
Build: NetBeans IDE Dev (Build 201112110600)
VM: Java HotSpot(TM) 64-Bit Server VM, 21.0-b17, Java(TM) SE Runtime Environment, 1.7.0-b147
OS: Windows 7
brettryan: Trying to remove project group.
jkovalsky: I had ReproduceNB Maven projects  opened and compiled. On the parent project called "Close Dependent Projects" and then "Close Project" on the remaining parent project generated this exception.
mhmdchebbi: happen when closing a maven project
GUEST: Closing a project
jportway: closing enterprise app maven project
carcassi: Changed active group
GUEST: Creating a project group from my currently opened projects. All projects are Maven projects.
sunbiz: Closing multiple projects throws this exception
RageSteel: I selected 7 project than right click on then and select
Created attachment 114035 [details]
Most reports are probably duplicates of bug #201617. Report #550435 is something else, I think due to unbalanced project open/close hooks from ergonomics. Cf. bug #203220 comment #9.
maybe a duplicate of issue 209815 ?
I do not think this is a duplicate; bug #209815 was a problem in project lookup whereas this seems related to ergonomics.
no more reports in post 7.2 builds, closing