Hello! I have the same problem over and over again with SMART 7.5 and I need to fix it 
When I try to upload I get an error “null”, and when I try to upload any other shape, the SMART desktop doesnt allow me anymore. It never give me the option to open again the basemap options. The only way to upload a shapefile its through the “+” option; but this layers I need tu upload every time I’ve open the app.
Im working with 3 CA, and I really appreciatte a solution 
@Tatiloove Can you please provide a little more context? Ideally, some screenshots clarifying?
It sounds like there maybe is an issue with your shapefiles, but it’s hard to say with current info.