How to solve this kind of errors?

Hi! guys!
          "Object reference not set to an instance of an object."
How to solve this kind of errors.In my application,am using C#.Net as codebehind,ASP.Net as front,VS2008(frmwrk3.5).
Usually what i'd do to solve this error is thatr i will clr my cache & Tempdatas in my browser.After that my prg will run good.But its not a professinal way right?.
error comes after this line
"DataSet DS = (DataSet1)Session["Content"];
                DataTable DT = DS.Tables[0];"
Regards,
   

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