Fix issue with accessing global attributes

[KristianKarl]

A fix for accessing global attributes

From the graphwalker forum: https://groups.google.com/g/graphwalker/c/o5y1_iE7F-o/m/Z9cg5OwZCQAJ

I'm prefixing my global variables with "global." and yet I'm unable to access them in other model implementation Java classes.
I get an error like this:
java.lang.NullPointerException: Cannot invoke "org.graalvm.polyglot.Value.asString()" because the return value of "com.example.modelimplementations.SecondViewTest.getAttribute(String)" is null

I added a commit to my graphwalker-problem GitHub repository that demonstrates this problem in code.

[KristianKarl]

Handling global attributes is a bit sketchy.
A java script added to the model with a global attribute and mixed with other variable names and functions called global. will mess things up.