It looks like we have regressed our handling of corrupted OpenStreetMaps in BC 4.5.0. Until the fix comes out, you will need to remove the corrupted map.
If you are consistently crashing, especially on startup, then it's likely you're using a corrupted OSM. You can either remove your maps one and a time (starting with OSM) until BaseCamp starts w/o crashing or look in the crash log. If you search on 'Activity:' in the crash log, then you'll find the failing map on the next line.
You can use MapManager to add and remove maps or if you are so inclined you can attempt the following steps:
To remove a map from a device:
1. Stop BaseCamp
2. Open Finder and look in your Garmin folder on the device.
3. The .img files are your maps. You can delete those, or just rename them using a different extension like .img.bad.
4. Start BaseCamp
To remove a map from BaseCamp:
1. Stop BaseCamp
2. Open Finder and select 'Go->Go To Folder'
3. Enter ~/Library/Application Support/Garmin/Maps
4. Delete the map you don't want or move it to another folder
5. Start BaseCamp again
Once you get BaseCamp up and running again, you can probably re-download your OSM and it will work just fine.
