Yes, if a field is not associated with a form and you call validateField on that then it return null,
https://github.com/joomla/joomla-cms/blob/staging/media/system/js/html5fallback-uncompressed.js#L141

The present codes between lines 140 to 142:

            if(elem.form === undefined){
                return null;
            }

So when the form value is null it does return null instead generates this error "Cannot read property 'H5Form' of null"

Therefore it should be replaced with

            if(elem.form === undefined || elem.form === null){
                return null;
            }

I guess elem.form === null is needed only. But to be on the safer side we can check for both.
Can you just open a tracker and send a PR towards staging branch, it would be great.

Yes, just did that. https://github.com/abhishekdas/joomla-cms/blob/patch-1/media/system/js/html5fallback-uncompressed.js#L140

PR: #3024

I am closing this in favour of rhe referenced #3024 which has a PR so we only discuss things in one issue