Object reference not set to an instance of an object.

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C#

i am using code for showing pps file and it will start file perfectly on local system but when i upload these on live server and acceess the page it will not starting the pss file and prompt me to save or open msg and i got "Object reference not set to an instance of an object" also what is the reason behind,Urgent issue .

protected void Button1_Click(object sender, EventArgs e)
    {





       
        Site = System.Diagnostics.Process.Start("http://elearning.aleaitsolutions.co.uk/TestAttachment/100pres.ppsx.pps").ToString();
        }

using Microsoft.Office.Core;
using Microsoft.Office.Interop.PowerPoint;
using ppt = Microsoft.Office.Interop.PowerPoint;
using System.Runtime.InteropServices;
using System.Diagnostics;
using System.ComponentModel;