createSession method
创建会话
Implementation
Session createSession({UserShip? usership, TeamShip? teamship}) {
if (usership != null) return Session.fromUserShip(usership, getUser(usership.rid));
if (teamship != null) return Session.fromTeamShip(teamship, getTeam(teamship.rid));
throw 'usership == null && teamship == null';
}