How do I resolve the "touppercase" javascript error message from the whutils.js when I click Search in a chm generated from the RH "WebHelp to CHM Converter.jsx" script?

I have successfully created and tested a skinned WebHelp output. However, when I run the chm generated from the RoboHelp script "WebHelp to CHM Converter.jsx" and click in the Search box, it immediately displays an error dialog with the description:

Unable to get property 'toUpperCase' of undefined or null reference

!The error is thrown from the whutils.js file, line 178.

I'm running RoboHelp 2017, v13.0.2.334

Your help as always is greatly appreciated.