I am trying to set up Ubuntu full disk encryption using preseeding. I was able to set things up correctly following the preseed file presented here. But I do not want to provide the default password in the preseed file. Is there a way to let ubiquity fall to the screen that asks for the encryption passphrase just like it does when the username is not provided in the preseed file? When I try to leave the passphrase option empty in the preseed file, I get an error in the installer saying "An error occured while creating the keyfile".