Empty mails are sent, error in /lib/pkp/classes/mail/Mailer.php\nLine: 79

After ending a new submission an empty E-Mail is sent, browser window doe not finish
If I want to finish a new submission I get an error, page hangs up, an empty E-Mail is sent.
But the submission is succesfull.

Error is:
get_class(): Argument #1 ($object) must be of type object, null given\nFile: .../lib/pkp/classes/mail/Mailer.php\nLine: 79

Using OJS 3.3.0-4