private void ReplaceWindow_Closed(object sender, EventArgs e)
 {
     _replaceWindow = null;
 }
        public void ReplaceCommand()
        {
            if (MainTextBox.Text.Length > 0)
            {
                if (null != _searchWindow)
                {
                    _searchWindow.Close();
                }

                if (null == _replaceWindow)
                {
                    _replaceWindow = new ReplaceWindow();
                    _replaceWindow.Owner = this;
                    _replaceWindow.Closed += ReplaceWindow_Closed;
                    _replaceWindow.Show();
                }
                else
                {
                    _replaceWindow.Activate();
                }
                this.MainMenuSubPopup.IsOpen = false;
            }
        }