You are missing validation on j itself. It seems in this case there is no DVD with the name you're looking for so ideally you should validate if j is NULL as well, instead of
if (j.Bringback==null)
Do the following
if (j != null || (j != null && j.Bringback==null))
So you are checking if the DVD exists, and if it does, the 2nd part of the validation is based on j itself, and if Bringback is null.