I Hope this will be helpfull.
Dbdesigner has a bug many person encounter. When this happens then there is no way to export the model to a database.
I did not find any help on the internet, nobody having found a good solution.
But after snooping around I found a solution I am happy to share .
Just stop dbdesigner4, discard (or rename if you are reluctanct) appdata/roamin/dbdesigner4 folder and restart dbdesigner.
Of course you will loose all gui ajustments, but the problem should be solved. If not it is a real circular reference!