App crash if station selected and boundary switch requested

If user of app has added an administrative boundary layer to the leaflet map, then selected a station (or more than one), and then **attempts to select a different administrative boundary before hitting the un-select stations button, the app crashes.**

Reproduce the crash in the Leaflet map by adding an administrative boundary, selecting one or more stations, and requesting a different boundary before unselecting the stations. Trace the station and boundary selection handling from that interaction. Done means switching boundaries no longer crashes and the map remains usable.

Written by the indexing model from the issue text.