Handle previewExpression element with no text

Author: manisandro

src/config_generator/qgs_reader.py

@@ -378,7 +378,8 @@ def __layer_edit_metadata(self, root, layer_metadata, maplayer, layername, map_p
 
         # Display field
         previewExpression = maplayer.find('previewExpression')
-        m = re.match(r'^"([^"]+)"$', previewExpression.text if previewExpression is not None else "")
+        if previewExpression is not None:
+            m = re.match(r'^"([^"]+)"$', previewExpression.text if previewExpression.text is not None else "")
         layer_metadata["displayField"] = m.group(1) if m else None
 
         # Generate form