Does someone know what this error is?

I'm getting this error: object reference not set to an instance.

after running this code

        DataTable dtOriginal = new DataTable();
        dtOriginal = (DataTable)gvRapporten.DataSource; //Return Table consisting data

        //Create Tempory Table
        DataTable dtTemp = new DataTable();

        //Creating Header Row
        dtTemp.Columns.Add("<b>Melder</b>");
        dtTemp.Columns.Add("<b>Onderwerp</b>");
        dtTemp.Columns.Add("<b>Oplosser</b>");
        dtTe开发者_如何学运维mp.Columns.Add("<b>Niveau 2</b>");
        DataRow drAddItem;
        for (int i = 0; i < dtOriginal.Rows.Count; i++)
        {
            drAddItem = dtTemp.NewRow();
            drAddItem[0] = dtOriginal.Rows[i][0].ToString();//Melder
            drAddItem[1] = dtOriginal.Rows[i][1].ToString();//Onderwerp
            drAddItem[2] = dtOriginal.Rows[i][2].ToString();//Oplosser
            drAddItem[3] = dtOriginal.Rows[i][3].ToString();//Niveau 2

            dtTemp.Rows.Add(drAddItem);
        }

The datasource of the gridview 'gvRapporten' is set at the page_load event, so it can't be NULL

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On a Post back the DataSource will be empty. You'll have to re-run the query to get the original data.

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If this code is not part of the 'page_load' then I assume that this is on the other control events which means the value of the gvRapporten.DataSource will be cleared on postback. Since web is 'State-less' it will not retain the datatable value, so you should get the value again from your database.

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If you are sure about gvRapporten, it is possible that one of the Rows accessed in the loop contains a NULL reference giving error when you call the ToString() function