PL/SQL XMLTYPE ORA-30625 - oracle

Question

I'm working with a web service that returns the output below. I'm having a hard time building the XMLTYPE variable from the output. I get the following error when trying to output the CreateUserSessionFromInstanceResult
ORA-30625: method dispatch on NULL SELF argument is disallowed
procedure xmltest is
str_xml varchar2(32000);
v_xml XMLTYPE;
BEGIN
str_xml :='<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<soap:Body>
<CreateUserSessionFromInstanceResponse xmlns="http://archer-tech.com/webservices/">
<CreateUserSessionFromInstanceResult>4FFABEE05C4910A31FEC75D5FEDCDFB5</CreateUserSessionFromInstanceResult>
</CreateUserSessionFromInstanceResponse>
</soap:Body>
</soap:Envelope>';
v_xml := XMLTYPE(str_xml);
HTP.P(v_xml.extract('//CreateUserSessionFromInstanceResult/text()').getstringval());
END;

Answer 1

The error is on this line
HTP.P(v_xml.extract('//CreateUserSessionFromInstanceResult/text()').getstringval());
You're getting an error because your XPath expression
//CreateUserSessionFromInstanceResult/text()
matches nothing, and so extract(...) returns NULL. Of course, you can't call getstringval() on NULL, hence the error you see.
The reason your XPath isn't matching is because of namespaces. The xmlns attribute on the <CreateUserSessionFromInstanceResponse> element sets the namespace of this element and of the <CreateUserSessionFromInstanceResult> element it contains to be "http://archer-tech.com/webservices/". On the other hand, your XPath expression is searching for the element with the name CreateUserSessionFromInstanceResult in the default namespace, "".
The fix is to add a third parameter to extract that declares this namespace. You do this in the same way as it appears in the XML:
HTP.P(v_xml.extract('//CreateUserSessionFromInstanceResult/text()',
'xmlns="http://archer-tech.com/webservices/"').getstringval());

Answer 2

I came across a similar case and adding the extra parameter didn't work for me.
For me, the solution was to check if node exists and do a null check before using getstringval, like this:
if v_xml.existsnode('//CreateUserSessionFromInstanceResult', 'xmlns="http://archer-tech.com/webservices/"') > 0 and v_xml.extract('//CreateUserSessionFromInstanceResult/text()', 'xmlns="http://archer-tech.com/webservices/"') is not null then
HTP.P(v_xml.extract('//CreateUserSessionFromInstanceResult/text()', 'xmlns="http://archer-tech.com/webservices/"').getstringval());
end if;
Doing only the null check is probably enough.