TypeError on label definition

[jusabatier]

I'm trying to define a simple label which is an aggregate of two fields, and when validate the style, I get an error from GeoCat Bridge plugin :

TypeError: cannot serialize None (type NoneType)
Traceback (most recent call last):
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 1052, in _escape_attrib
if "&" in text:
TypeError: argument of type 'NoneType' is not iterable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins/geocatbridge/ui/styleviewerwidget.py", line 48, in updateLayer
self.updateForCurrentLayer()
File "/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins/geocatbridge/ui/styleviewerwidget.py", line 58, in updateForCurrentLayer
sld, _, sld_warnings = layerStyleAsSld(layer)
File "/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins/geocatbridge/libs/bridgestyle/bridgestyle/qgis/init.py", line 15, in layerStyleAsSld
sldString, sldWarnings = sld.fromgeostyler.convert(geostyler)
File "/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins/geocatbridge/libs/bridgestyle/bridgestyle/sld/fromgeostyler.py", line 72, in convert
sldstring = ElementTree.tostring(root, encoding="utf8", method="xml").decode()
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 1109, in tostring
ElementTree(element).write(stream, encoding,
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 748, in write
serialize(write, self._root, qnames, namespaces,
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 913, in _serialize_xml
_serialize_xml(write, e, qnames, None,
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 913, in _serialize_xml
_serialize_xml(write, e, qnames, None,
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 913, in _serialize_xml
_serialize_xml(write, e, qnames, None,
[Previous line repeated 4 more times]
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 906, in _serialize_xml
v = _escape_attrib(v)
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 1075, in _escape_attrib
_raise_serialization_error(text)
File "/usr/lib/python3.9/xml/etree/ElementTree.py", line 1029, in _raise_serialization_error
raise TypeError(
TypeError: cannot serialize None (type NoneType)
Version de Python : 3.9.5 (default, May 11 2021, 08:20:37) [GCC 10.3.0]
Version de QGIS : 3.20.1-Odense (Odense 2755615b9f)
Python path:
/usr/share/qgis/python
/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python
/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins
/usr/share/qgis/python/plugins
/usr/bin/python3.10
/usr/lib/python39.zip
/usr/lib/python3.9
/usr/lib/python3.9/lib-dynload
/home/jusabatier/.local/lib/python3.9/site-packages
/usr/local/lib/python3.9/dist-packages
/usr/lib/python3/dist-packages
/usr/lib/python3.9/dist-packages
/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python
/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins/cadastre/forms
/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins/qgis_resource_sharing
/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins/qgis_resource_sharing/ext_libs
/home/jusabatier/.local/share/QGIS/QGIS3/profiles/default/python/plugins/geocatbridge/libs/bridgestyle

I simply define for a point layer a style with label ans background for it.

Acording comment in GeoCat/qgis-bridge-plugin#132 , I post this issue here as it's seems to be related with styling.