Does the word "validator" exist?
According to n-gram it's been around for about a century, and getting more popular all the time.
Well the word definitely exists. I think its origin would be technical, eg. it is heavily used in software engineering. I'm not sure it's made the leap into common usage for "something or someone that validates ...". That said, I'm sure you would be understood.
As a double-check: what's your thesis about? Would the word normally be used in that context?