it is needed to be validateField because if we do validation of all field on the change of only one then we can have a lag if number of fields are present.
That error we are getting because chosen added an anchor tag which is firing focusout event and no form is associated with the anchor tag. So since no form is associated then that means it doesn't need to be validated. It's solution is
https://github.com/Achal-Aggarwal/joomla-cms/commit/d5b0723b1942cc752dea1a78ad6839fded021b08