Take the following example protocol:

<?xml version="1.0" encoding="utf-8"?>

<xcb header="bad" extension-name="bad" extension-xname="BAD">
        <request name="BadRequest" code="0">
                <field type="CARD32" name="sizeField" />
                <list type="CARD8" name="dataVector">
                        <fieldref>vectorSize</fieldref>
                </list>
        </request>
</xcb>

When running it through XCB's codegen, the result is a backtrace and a TypeError that NoneType can't be concatenated with 'str'. As it stands, the error gives no hint that the issue is that vectorSize isn't a previously defined field in the request, which can be easily overlooked(especially if the error is a simple shift failure). Not having any experience in Python, I can't suggest what would be the best fix, but it looks like adding a check in get_type_impl(state.py) for id == NoneType should be sufficient for a simple "Check your fieldrefs" diagnostic.