Fix Call to a member function only() on null error

If a participant is deleted and you try to get the messages in a conversation you will get an error when trying to get the sender attribute. This fixes it.

Author: pajkho

src/Models/Message.php

@@ -47,6 +47,10 @@ public function participation()
     public function getSenderAttribute()
     {
         $participantModel = $this->participation->messageable;
+        
+        if (!isset($participantModel)){
+                return null;
+        }
 
         if (method_exists($participantModel, 'getParticipantDetails')) {
             return $participantModel->getParticipantDetails();