Don't expect messages to always have text
Some messages are notices that don't carry any text (responses to forum invitations for example).
Traceback (most recent call last):
File "/usr/share/briar-gtk/briar_gtk/containers/private_chat.py", line 101, in _add_message
message_widget = PrivateMessageWidget(self._contact_name, message)
File "/usr/share/briar-gtk/briar_gtk/widgets/private_message.py", line 18, in __init__
self._setup_view(contact_name, message)
File "/usr/share/briar-gtk/briar_gtk/widgets/private_message.py", line 34, in _setup_view
message["text"])
KeyError: 'text'