Issue working with Groovy scripts

I'm working through the very first 'Getting Started' tutorial and having an issue when entering the scripts for constraints. I consistently get 'An internal error occurred during: "Unknown elements indexer"' error. It lists Constraint inputs indexer and unknown elements indexer and provides the following detail:
An internal error occurred during: "Unknown elements indexer".
java.lang.StackOverflowError

the .log file contains the following:
!ENTRY org.eclipse.core.jobs 4 2 2018-04-03 15:34:10.996
!MESSAGE An internal error occurred during: "Unknown elements indexer".
!STACK 0
java.lang.StackOverflowError
at java.util.HashMap$TreeNode.find(HashMap.java:1859)
at java.util.HashMap$TreeNode.find(HashMap.java:1869)
at java.util.HashMap$TreeNode.find(HashMap.java:1869)
at java.util.HashMap$TreeNode.find(HashMap.java:1869)
at java.util.HashMap$TreeNode.find(HashMap.java:1869)
at java.util.HashMap$TreeNode.find(HashMap.java:1869)
(last line just keeps repeating, making it appear as if it's stuck in a loop)

Sometimes I can enter a few constraints before it comes up, other times it starts pretty much immediately. As soon as it happens once, I can click ok to go back to my script but typing any further character in the script window immediately causes the error again. Not sure what I can really try to rectify this. I'm using Bonita CE 7.6.3 running on Windows 10 Enterprise edition. I have Java 8u162 64 bit installed as well as Java 8u161 32 bit.