Deprecated element

Hello,

I just installed your plugin in a website using Blocksy theme.
When I activated it, I had an alert saying it could not be activated, although it was. Switched on debug mode to check if any problem was mentionned and no error message. (at least not on all pages)
then I added a first term, displayed it, and used the given link to Glossary index page, and there I had this message :

Deprecated: substr(): Passing null to parameter #1 ($string) of type string is deprecated in?/home/cclga64/public_html/tap/wp-content/plugins/enhanced-tooltipglossary/glossaryIndex.php?on line?1458

May you help ?

thanks

The page I need help with: [log in to see the link]