Why listbox applicaion crashes on exit?

HI,
I am beginner to symbian. i have created a simple application that allow the user to logon,

after successfull login, it display the listbox, i created login dialog using resource file

if it return OK then i my Appui class create the Listboxcontainer object. and perform

actions, but the problem is that when user click back or exit while on Listbox screen, it

crashes.Error "Program closed". the main function in Apui ConstructL is :

void CMKISAppAppUi::ConstructL()
{
BaseConstructL();

TBool okPressed = CMKISAppLoginContainer::RunDlgLD(); // call login Dialog
if (okPressed)
{
iKeyListContainer = new (ELeave) CMKISAppKeyListContainer;
iKeyListContainer->SetMopParent( this );
iKeyListContainer->ConstructL( ClientRect() );
AddToStackL( iKeyListContainer );
}
else
{
Exit();
}
}

CMKISAppAppUi::~CMKISAppAppUi()
{
if (iKeyListContainer)
{
RemoveFromStack( iKeyListContainer );
delete iKeyListContainer;
}
}
void CMKISAppAppUi::HandleCommandL(TInt aCommand)
{
switch ( aCommand )
{
case EEikCmdExit:
{
Exit();

break;
}
.......
}

please help me,
any suggestion will be highly appreciated.
Thanx in advance.