I think I might know how to fix it very easily, but I don’t know what’s the cause though.
Open the Compatibility Settings submenu under the ajax search pro menu, and hit the Save button without changing anything. The error refers to unset variables regarding the compatibility options, and this should refresh the values.
I’ve checked the lines the error message is referring to, but I need to run some tests why the error message is triggered, as I have no idea yet.