PHP 7.4 starts issuing error ‘warnings’ if a variable doesn’t exist and you use a statement like your example. Code still works, though, although the error warnings are in your error.log file.
I haven’t updated that plugin for a while. Instead, for my use case, I just built a bare-bones template based on my theme. So I am not surprised that there are issues. Probably even more in PHP 8+.
I might try to update the plugin for PHP 8.x, which will take care of those errors. But I have no ETA, due to other projects that have a bit more demand (I’m a one-person shop). If I do find time, it may take at least a month to squeeze it into my schedule.
Your issue has made me more aware of the need to update the plugin for PHP 8, though.