Hello,
I have a problems when I want to get the groupname:
Error: https://gyazo.com/444e7482b6bcb7be15e2a8864dc560c7
Code: https://gyazo.com/60eefb84f47f6e2b99d0c9b5531b499e

You need to check Node.isGroupNode() before calling Node.getGroupName().

I've fixed the class cast exception in the commit above, so the proper error message will be printed. :)