Fix null exception about initial

[laserdark]

For the function of BossSettings.MapItemsSource, it seems to be with same output from either Models.Maps.MapInfos or KanColleClient.Current.Master.MapInfos sources. While the initial trigger is related to KanColleClient.Current.IsStarted, and at same time Models.Maps.MapInfos hasn't been initialized already, it couldn't be a bad way to change origin data from Models.Maps to KanColleClient.Current.