C# – Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate

i have a gridview that creates a new row with a new gridview in it
the method of creating the second gridview is :

protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
{
    if (e.CommandName == "PopExtendedGrid")
    {


    GridView _gridView = (GridView)sender;
int _rowIndex2 = int.Parse(e.CommandArgument.ToString());
GridView _ChildGrid = new GridView();
Control x = _gridView.Rows[_rowIndex2 + 1].Cells[1].Controls[1];
int Oid = int.Parse(((Label)x).Text);
_ChildGrid.DataSource = hs.GetExtendedGrid(Oid);
_ChildGrid.ID = "ChildGrid";
_ChildGrid.AutoGenerateColumns = false;
_ChildGrid.CssClass = "ChildGridS";
_ChildGrid.HeaderStyle.CssClass = "CreateHead";
BoundField one = new BoundField();
one.DataField = "ItemID";
one.HeaderText = "קוד מוצר";
_ChildGrid.Columns.Add(one);

BoundField two = new BoundField();
two.DataField = "ItemName";
two.HeaderText = "שם מוצר";
_ChildGrid.Columns.Add(two);

BoundField three = new BoundField();
three.DataField = "ItemSize";
three.HeaderText = "גודל מוצר";
_ChildGrid.Columns.Add(three);

GridViewRow tr = new GridViewRow(_rowIndex2 + 2 +10*this.GridView1.PageIndex,-1 , DataControlRowType.EmptyDataRow , DataControlRowState.Normal);
TableCell tc = new TableCell();
tc.ColumnSpan = _gridView.Columns.Count;
tc.Controls.Add(_ChildGrid);
tr.Cells.Add(tc);
if ((DataView)Session["dataSource"] != null)
 {
 DataView dv = (DataView)Session["dataSource"];
 this.GridView1.DataSource = dv;
 this.GridView1.DataBind();
 }
                else
                {
                    if (Session["lvl"].ToString() == "high")
                    {
                        PopulateGridViewAdmin();
                    }
                    else
                    {
                        PopulateGridViewUser();
                    }
                }
this.GridView1.Controls[0].Controls.AddAt(_rowIndex2 + 2, tr);
Session["ChildIndex"] = (_rowIndex2 + 2).ToString();
_ChildGrid.DataBind();
  }
}

and i thought that when ever i will need to execute another command or something that is related to the gridview i will remove the row like this:

this.GridView1.Controls[0].Controls.RemoveAt(int.Parse(Session["ChildIndex"].ToString()));

and then repopulate the mastergridview but before i get the chance to do so this error keeps poping up:

Failed to load viewstate.  The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request.  For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.

im very frustrated by this and i will take any answer with great gratitude
thanks in advance