初学C#，请问如何在一个窗体用ToolStripMenuItem_Click控制另一个子窗体的打开关闭，并且焦点不变。

目前已经实现了通过Click事件开关子窗体，但是一开就变了，而且开了之后如果用ToolStripMenuItem的checked属性来控制开关没问题，可是手动关闭了子窗体后，再点击，就会抛出异常未处理 
System.ObjectDisposedException
  Message="无法访问已释放的对象。\r\n对象名:“Form2”。"
  Source="System.Windows.Forms"
  ObjectName="Form2"
  StackTrace:
       在 System.Windows.Forms.Control.CreateHandle()
       在 System.Windows.Forms.Form.CreateHandle()
       在 System.Windows.Forms.Control.get_Handle()
       在 System.Windows.Forms.Control.SetVisibleCore(Boolean value)
       在 System.Windows.Forms.Form.SetVisibleCore(Boolean value)
       在 System.Windows.Forms.Control.Show()
       在 ActionEditer.Form1.打开预览窗口() 位置 E:\Projects\VS Projects\ActionEditer\ActionEditer\Form1.cs:行号 28
       在 ActionEditer.Form1.动画ToolStripMenuItem_Click(Object sender, EventArgs e) 位置 E:\Projects\VS Projects\ActionEditer\ActionEditer\Form1.cs:行号 154
       在 System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e)
       在 System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e)
       在 System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e)
       在 System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)
       在 System.Windows.Forms.ToolStripItem.FireEventInteractive(EventArgs e, ToolStripItemEventType met)
       在 System.Windows.Forms.ToolStripItem.FireEvent(EventArgs e, ToolStripItemEventType met)
       在 System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea)
       在 System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)
       在 System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       在 System.Windows.Forms.Control.WndProc(Message& m)
       在 System.Windows.Forms.ScrollableControl.WndProc(Message& m)
       在 System.Windows.Forms.ToolStrip.WndProc(Message& m)
       在 System.Windows.Forms.ToolStripDropDown.WndProc(Message& m)
       在 System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       在 System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       在 System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       在 System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       在 System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
       在 System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       在 System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       在 System.Windows.Forms.Application.Run(Form mainForm)
       在 ActionEditer.Program.Main() 位置 E:\Projects\VS Projects\ActionEditer\ActionEditer\Program.cs:行号 18
       在 System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
       在 System.AppDomain.nExecuteAssembly(Assembly assembly, String[] args)
       在 System.Runtime.Hosting.ManifestRunner.Run(Boolean checkAptModel)
       在 System.Runtime.Hosting.ManifestRunner.ExecuteAsAssembly()
       在 System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext, String[] activationCustomData)
       在 System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext)
       在 Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssemblyDebugInZone()
       在 System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       在 System.Threading.ThreadHelper.ThreadStart()
  InnerException: -----------------------------------------------------
以下是我的代码
-----------------------------------------------------        public Form2 fm2;
        public Form1()
        {
            InitializeComponent();
            this.IsMdiContainer = true;
            fm2 = new Form2();
            this.打开预览窗口();
        }
        private void 打开预览窗口()
        {
            if (fm2 != null)
            {
                fm2.Show();
            }
        }        private void 关闭预览窗口()
        {
            fm2.Hide();
        }
        private void 动画ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.动画ToolStripMenuItem.Checked = !this.动画ToolStripMenuItem.Checked;
            if (this.动画ToolStripMenuItem.Checked)
            {
                this.打开预览窗口();
            }
            else
            {
                this.关闭预览窗口();
            }
        }我最终想实现的一个功能就像是PhotoShop里面那些图层，信息之类的子窗体的控制一样。
恳请各位大哥帮帮忙