Messaging to seller

When i opened page "http://applicationaddress.com/messaging/discussions/create?user=4ndKGRzMmZ" Then Submit form, there was an error like this: Argument 1 passed to Corals\Modules\Messaging\Models\Discussion::addParticipant() must be an instance of Illuminate\Database\Eloquent\Model, null given, called in C:\Users\gritjamPC\Documents\_project\psrfn\Corals\modules\Messaging\Models\Discussion.php on line 298   How do i resolve this problem? thanks