Fatal error: Can’t use function return value in write context in …

Very strange error, what php version are you using?
The line https://plugins.trac.wordpress.org/browser/glossary-by-codeat/trunk/public/includes/Glossary_Search_Engine.php#L144 is about a check if the post has a title but this method should be not execute because happen only when the plugin is executed in a post.
Are you getting that error when active the plugin or after that the plugin is activated in another page?

Happy that the problem is fixed, we will do few tests in old environment soon just to be sure if it is a problem of the plugin 🙂