I had the same and checked metalink : As developer 6 does not support multiple homes, it has to be installed before the 816 database software.

If you already created the database abd do not want to do that again, shut down th edatabase and make a backup of all datafiles, controlfile etc. Remove all oracle software, developer and rdbs , delete the oracle registry keys and the oracle services and reinstall developer. Afterwards reinstall 816 and recreate the service for the database (oradim). Copy back the datafiles etc and you will be able to start up the database.

Good luck