title: getting system.ArgumentNullException on FirstOrDefault()
body: <p>As I cant add the whole code here i am posting justa snippet.
  I am getting and exception "System.ArgumentNullException"and an error that can't implicitly convert from Ins_ProjectTestCycleDet_Result to Project_List. I have started getting this error when i added last argument as null (after ST). Please help me with knowing what exactly is the issue as even in the database i have set this particular field as Allow null.</p>
  
  <pre><code>Project_List objProject_List = new Project_List();
  objProject_List.Proj_Bill_ID = Convert.ToInt32(PID);
  
  objProject_List = _OCB.Ins_ProjectTestCycleDet
                     ( Convert.ToInt32(PID), TID, RID, ECY
                     , TT, Convert.ToDateTime(GD), Convert.ToDateTime(SD)
                     , Convert.ToDateTime(EED), Convert.ToDateTime(ED)
                     , null, Convert.ToDecimal(TP), Convert.ToDecimal(TE)
                     , Convert.ToDecimal(TDA), Convert.ToDecimal(TR), TCR, ST, null
                     ).FirstOrDefault();
  </code></pre>