I got "Document must not contain null namespace" error?

Well yeah. Schematic is based on namespaces - it chooses schema files based on the information provided by namespaces. So, if document contains at least one element with null namespace then Schematic cannot decide which schema to use to validate contents of such element.

To get rid of the problem just declare a correct namespace for the document. For example, for namespaceless DocBook XML just add xmlns="http://docbook.org/ns/docbook" attribute to root element.