Well, from the error message you are apparently (on line 203 of class.mailing.php) trying to create a default object from an empty value.
What that means is usually that you have a line like
$a->b = "c"; but
$a wasn't set to any value. Which means there's a problem with the code somewhere.
I'm guessing from the filepath that you're using Joomla! and Acajoom. The fault lies in that code (unless the problem is you're giving it bad data — it's impossible to say given how unhelpful your description of the problem is — but even then Acajoom's developers (i.e. Joobi) should be doing something about the possibility).