tab control

Question

hi, i am using tabcontrol project and there is a button in one of the tab page and when the user clicks the button a new window is opened My problem is that when i close the window and change tab an error occurs The error listing is shown below. Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object. at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg) at System.Windows.Forms.ComponentManager.System.Windows.Forms.UnsafeNativeMet hods+IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, In t32 pvLoopData) at System.Windows.Forms.ThreadContext.RunMessageLoop(Int32 reason, Applicatio nContext context) at System.Windows.Forms.Application.RunDialog(Form form) at System.Windows.Forms.Form.ShowDialog(IWin32Window owner) at System.Windows.Forms.Form.ShowDialog() at System.Windows.Forms.ThreadContext.OnThreadException(Exception t) at System.Windows.Forms.Control.WndProcException(Exception e) at System.Windows.Forms.ControlNativeWindow.OnThreadException(Exception e) at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg) at System.Windows.Forms.ComponentManager.System.Windows.Forms.UnsafeNativeMet hods+IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, In t32 pvLoopData) at System.Windows.Forms.ThreadContext.RunMessageLoop(Int32 reason, Applicatio nContext context) at System.Windows.Forms.Application.Run(Form mainForm) at Form1.Main(String[] args) please help me thanks in advance msfan