Just two small alterations:

• Previously, pylons used to return empty string('') for any undefined value, so let's use the same, instead of None
• We are replacing c global variable with g - they are pretty same, but latter is more Flask style, so could you switch to it as well?

Hi @smotornyuk, thank you for your suggestions.
I can use an empty string instead of None, but switching from c to g would not fix the issue. I tried to use g.user instead of c.user without any additional code, but it fails. I tried to setup g.user and then use c.user, but it still fails. The only way I found to avoid this issue is just to set c.user.

No, I'm perfectly ok with your solution and switching to g didn't suppose to fix the problem, because g and c are the same, as I said before. You just should use g instead of c in the feature, because c - it's how Pylons calls global context, and g - is a way of naming it from Flask. As we are migrating to Flask, I just want to use flask-style names for variable
https://github.com/ckan/ckan/wiki/Migration-from-Pylons-to-Flask#c-object

Thank you for the clarification. I did not understand that they were equivalent and I was confused by the fact that using g.user is not enough, even if the previous patch set it.

I had to import g, as helpers.py uses c only.