Fix for a situation that should be impossible

assets/models.py

@@ -148,7 +148,7 @@ class Asset(models.Model, RevisionMixin):
 
     def __str__(self):
         out = str(self.asset_id) + ' - ' + self.description
-        if self.is_cable:
+        if self.is_cable and self.cable_type is not None:
             out += '{} - {}m - {}'.format(self.cable_type.plug, self.length, self.cable_type.socket)
         return out