How did I cause a stack corrouption?

#pragma once
#include "CEGUI_Includes.h"

class GUIGroup
{
private:
	virtual void setupEventHandlers() = 0; // bind events to controls

protected:	
	CEGUI::Window * GroupWindow_;	 // parent window of the UI group
	
public:
	virtual ~GUIGroup(void);		 // release all resources

	virtual void onLoad(void) = 0;	 // initalise the GUI and its controls
	virtual void onHide(void) = 0;	 // called when GUI group is switched for another
	virtual void onShow(void) = 0;   // called when the GUI group is showed

	void addTo( CEGUI::Window*&  dest); // add window holding all the UI to given window
	void load(void);				   // load the gui group
	void show(void);				   // display the window
	void hide(void);				   // hide the window

	// return the window of the group
	CEGUI::Window* GetWindow(void)
	{
		return GroupWindow_;
	
	}
};

#include "GUIGroup.h"

// Release all acquired resources
GUIGroup::~GUIGroup(void)
{
	if (GroupWindow_)
	{
		delete GroupWindow_;
		GroupWindow_ = 0;
	}
}


void GUIGroup::addTo(CEGUI::Window*& dest)
{
	dest->addChildWindow(this->GroupWindow_);
}

// Load the GUI group
void GUIGroup::load()
{
	this->onLoad();
}

// Show the GUI group
void GUIGroup::show()
{
	onShow();
	this->GroupWindow_->show();
}

// Hide the GUI group
void GUIGroup::hide()
{
	this->GroupWindow_->hide();
	onHide();
}

#ifndef SAMPLE_MENU
#define SAMPLE_MENU

#include "GUIGroup.h"

/*
A sample GUIGroup to play with
*/
class CSampleMenu : public GUIGroup
{
public:
	
	~CSampleMenu(void);

	void onLoad(void);	   // initalise the GUI and its controls
	void onHide(void) {};  // called when GUI group is switched for another
	void onShow(void) {};  // called when the GUI group is showed

private:
	void setupEventHandlers() {};	
};

#endif

#include "SampleMenu.h"

void CSampleMenu::onLoad()
{
	// Load in the imagesets
	CEGUI::ImagesetManager::getSingletonPtr()->createImageset ("MenuButtons.imageset", "General");
	CEGUI::ImagesetManager::getSingletonPtr()->createImageset ("MenuBackground.imageset", "General");

	// Load in the layout
	GUIUtil::loadLayout("Menu.layout", &GroupWindow_ );	
}