You seem to have a global variable or parameter that tries to access the
global context item, so far you have only set the initial-match
selection, use e.g.

B xdm_doc = proc.parse_xml(xml_file_name='nav-short.xml')

and then set both the initial match selection as well as the global
context item to that e.g.

B executable.set_initial_match_selection(xdm_value=xdm_doc)

B executable.set_global_context_item(xdm_item=xdm_doc)

Or use
https://www.saxonica.com/saxon-c/doc12/html/saxonc.html#PyXsltExecutable-tran
sform_to_value
e.g.