contentpresenter checkbox casting error

wpf System.Windows.controls.contentpresenter checkbox casting error

       FrameworkElement element = this.dataGrid1.Columns[4].GetCellContent(dataGrid1.Items[Row_num]);
                    object chk = FindVisualChild<CheckBox>(element);
                    if (chk != null)
                    {
                        CheckBox use2 = chk as CheckBox;
                        if (use2.IsChecked ?? false)
                        {
                            MessageBox.Show("update1");
                        }
                        else
                        {
                            MessageBox.Show("update2");
                        }

                    }

     private childItem FindVisualChild<childItem>(DependencyObject obj)
           where childItem : DependencyObject
        {
            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
            {
                DependencyObject child = VisualTreeHelper.GetChild(obj, i);
                if (child != null && child is childItem)
                    return (childItem)child;
                else
                {
                    childItem childOfChild = FindVisualChild<childItem>(child);
                    if (childOfChild != null)
                        return childOfChild;
                }
            }
            return null;
        }