Bugfix for #102 - add additional checks for null system.
Bugfix for #102 - add additional checks for null system.

Possibly a better fix would be to make the "Distance" column read-only unless a valid system has been entered into the first column.

mwerle/EDDiscovery,jeoffman/EDDiscovery,jeoffman/EDDiscovery,mwerle/EDDiscovery,vendolis/EDDiscovery,jthorpe4/EDDiscovery,jgoode/EDDiscovery,jgoode/EDDiscovery,vendolis/EDDiscovery