user: initialize _latest_non_transient_bridge_state

Signed-off-by: Sumner Evans <sumner@beeper.com>

mautrix_signal/user.py

@@ -91,6 +91,7 @@ class User(DBUser, BaseUser):
         self._connected = False
         self._state_id = self.username
         self._websocket_connection_state = None
+        self._latest_non_transient_bridge_state = None
         perms = self.config.get_permissions(mxid)
         self.relay_whitelisted, self.is_whitelisted, self.is_admin, self.permission_level = perms