GridView paging takes two clicks instead of one

public static DataTable dt; // to store data loaded from another server

...

protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
       {
          lock (lockObject)
          {
             GridView1.EditIndex = e.NewEditIndex;
             GridView1.DataSource = dt;
             GridView1.DataBind();   
          }        
       }
...
protected void btnCheckLdap_click(object sender, EventArgs e)
        {
          lock (lockObject)
          {
                if (dt == null)
                {
                    dt = new DataTable(); 
                  //populate DataTable - this can take much time ~ 2-3 seconds
                     ...
                }
                GridView1.DataSource = dt;
                GridView1.DataBind();
          }  
        }
...

 <asp:GridView ID="GridView1" runat="server" ShowHeaderWhenEmpty="True" 
                    CssClass="GridView" AutoGenerateColumns="false" 
                    OnRowEditing="GridView1_RowEditing" OnRowUpdating="OnRowUpdating" 
                    onRowCancelingEdit="GridView1_RowCancelingEdit" EnableViewState="true" 
                    AllowPaging="true" AllowSorting="true" 
                    onpageindexchanging="GridView1_PageIndexChanging" 
                    onselectedindexchanged="GridView1_SelectedIndexChanged">
...