i had this issue, and this fix worked (deleting the db). it worked for a while, then i changed a file name and it happened again. i tried changing the file back, that didn't solve it. then i tried the steps here again (removed the installation files, dll, db, etc; checked it wasn't listed in the config; reinstalled everything) this time this fix isn't working.
the db is gone, so i don't know why it is still giving me this SelectedIndex error :/
(edit)
seemed to have fixed it. i launched MP, the db reappeared. closed MP. tried config again, still crashed. then i deleted the db (again), tried config again and it seems to have solved it (for now)