Certain characters cause exception when type in editor

Greetings,

I have been working on a module suite as a project for school that
enables writing R programs in NetBeans. The following characters cause
a NullPointerException when typed in the editor with the plugin
running: ' " _ . % \

I used ANTLR4 version 4.5.3 and the R.g4 and RFilter.g4 grammar files
from ANTLR's GitHub site to generate the lexer and parser.

I can cut and paste R code from an external file and execute the code
from NetBeans without issue.

I suspect the issue may be caused by the grammar file(s) or the
generated lexer, but I'm not sure where to start. Any guidance
appreciated.

Respectfully,

Steve