[xmlsec] ErrorsCallback ???

[Aleksey Sanin]

1) Are you using xmlsec directly or via python? If you have C/C++ app,
simply set a  breakpoint in the error callback and then trace it back
to the error origin. You'll see the exact place where NULL shows up.

2) The default callback sets the reported error in to libxml2 thus
you can use xmlGetLastError() directly. If you have a custom callback
then you can do similar thing yourself.

Aleksey