Thanks.
Uncaught (in promise) TypeError: Cannot read property 'fileName' of null
and
Uncaught (in promise) TypeError: db is null
are the same error - it’s just that Firefox does a better job of describing it than Chrome. It’s really not telling us much we don’t already know though - for whatever reason Kee is not receiving a database from KeePass.
Might be worth seeing if there are any useful error messages from KeePass or KeePassRPC log too - Troubleshooting