Unsuccesful gateway subscription raises AttributeError: 'NoneType' object has no attribute 'getTag'

Versions

• OS: Debian buster
• Gajim version: flatpak 1.3.0

Steps to reproduce the problem

1. Try to register to a gateway
2. Enter bad credentials

Expected behavior

Gajim should display the error message from the gateway component.

Actual behavior

The gateway registration window cannot be closed and is indefinitely in a "loading" state.

Traceback (most recent call last):
  File "/app/lib/python3.8/site-packages/nbxmpp/dispatcher.py", line 433, in dispatch
    func(self._client, stanza, **user_data)
  File "/app/lib/python3.8/site-packages/gajim/common/modules/register.py", line 75, in _register_agent_response
    form, is_form = self._get_register_form(stanza)
  File "/app/lib/python3.8/site-packages/gajim/common/modules/register.py", line 112, in _get_register_form
    parse_bob_data(stanza.getQuery())
  File "/app/lib/python3.8/site-packages/gajim/common/modules/bits_of_binary.py", line 105, in parse_bob_data
    data_node = stanza.getTag('data', namespace=Namespace.BOB)
AttributeError: 'NoneType' object has no attribute 'getTag'

Relevant XML console content

<!-- Outgoing ven. 12 févr. 2021 10:14:46 (lolo@localhost) -->
<iq xmlns="jabber:client" to="signald.localhost" type="set" id="90feed3e-fcc0-426b-8c72-f33079d56875">
  <query xmlns="jabber:iq:register">
    <username>sdfsdf</username>
  </query>
</iq>


<!-- Incoming ven. 12 févr. 2021 10:14:46 (lolo@localhost) -->
<iq xmlns="jabber:client" type="error" id="90feed3e-fcc0-426b-8c72-f33079d56875" from="signald.localhost" to="lolo@localhost/gajim.23K229O1">
  <query xmlns="jabber:iq:register">
    <username>sdfsdf</username>
</query>
  <error code="406" xmlns="jabber:client" type="modify">
    <not-acceptable xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
    <text xmlns="urn:ietf:params:xml:ns:xmpp-stanzas">This does not look like a valid phone number</text>
</error>
</iq>