有高手遇到这种情况吗，请教

System.NullReferenceException: 未将对象引用设置到对象的实例。
   在 App.frmMain.MenuSecond_Click(Object sender, EventArgs e)
   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.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)