title: DataTable.Merge throws Error: Object reference not set to an instance
body: <p>I want to merge datatables in the DropDownList selectedindex changed event. So I have written the below code in the DropDownList_SelectedIndexChanged Event. </p>
  
  <pre><code>    DataTable dt = (DataTable)POBLL.GetPOAdhocDetail();
      DataView dv = new DataView(dt);
      dv.RowFilter = "IndentTypeID = " + ddlIndentType.SelectedValue + "AND IndentID = " + ddlIndentNumber.SelectedValue;
      DataTable dtView = dv.ToTable();        
      gvPODetails.DataSource = dtView;
      gvPODetails.DataBind();
      DTMerge.Merge(dtView);
  </code></pre>
  
  <p>where DTMerge is the DataTable and I declared it like below:</p>
  
  <pre><code>   public DataTable DTMerge
     {
        get
        {
          if (ViewState["Merge"] != null)
          {
              return (DataTable)ViewState["Merge"];
          }
          else
              return null;
        }
        set
        {
           ViewState["Merge"] = value;
        }
    }
  </code></pre>
  
  <p>But it throws the Error "Object reference not set to an instance". How to solve this?</p>