I have an XML file which looks like this:
<testrequest version="1.0">
  ...
</testrequest>
and I have a suitable XML Schema. To validate the file, I used the menu XML/Associate/XML Schema and entered the file name.
After this, XML Editor modified (!) my XML file to:
<testrequest version="1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nonamespaceschemalocation="....">
  ....</testrequest>
This is of course invalid, since the topmost element does not permit a xmlns:xsi attribute!
Sorry, my mistake. I just verified that, since <testrequest> is the topmost element, the schema location MUST go there. Please close the ticket.</testrequest>
Closed as the reporter requested.