Unhandled exception when closing directx

[sharksaw40]

I am learning directx from directxtutorial.com and when I have compiled one of the samples and run it then close it I get an error message saying "Unhandled exception at 0x010a1883 in test.exe: 0xC0000005: Access violation reading location 0x00000000.". What does this mean? I have checked that I have typed out all the code correctly and that it's exactly the same as on the tutorial. Here it is:

#include <windows.h>
#include <windowsx.h>
#include <d3d11.h>
#include <d3dx11.h>
#include <d3dx10.h>

#pragma comment (lib, "d3d11.lib")
#pragma comment (lib, "d3dx11.lib")
#pragma comment (lib, "d3dx10.lib")

IDXGISwapChain *swapchain;
ID3D11Device *dev;
ID3D11DeviceContext *devcon;

void InitD3D(HWND hWnd);
void CleanD3D(void);

LRESULT CALLBACK WindowProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
	HWND hWnd;

	WNDCLASSEX wc;

	ZeroMemory(&wc, sizeof(WNDCLASSEX));

	wc.cbSize = sizeof(WNDCLASSEX);
	wc.style = CS_HREDRAW |CS_VREDRAW;
	wc.lpfnWndProc = WindowProc;
	wc.hInstance = hInstance;
	wc.hCursor = LoadCursor(NULL, IDC_ARROW);
	wc.hbrBackground = (HBRUSH)COLOR_WINDOW;
	wc.lpszClassName = "WindowClass1";

	RegisterClassEx(&wc);

	RECT wr = {0,0,800,600};

	AdjustWindowRect(&wr, WS_OVERLAPPEDWINDOW, FALSE);

	hWnd = CreateWindowEx(NULL, "WindowClass1", "Our first windowed program", WS_OVERLAPPEDWINDOW, 300, 100, wr.right - wr.left, wr.bottom - wr.top, NULL, NULL, hInstance, NULL);

	ShowWindow(hWnd, nCmdShow);

	InitD3D(hWnd);

	MSG msg;

	ZeroMemory(&msg, sizeof(MSG));

	while(TRUE)
	{
		if(PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
		{
			TranslateMessage(&msg);

			DispatchMessage(&msg);

			if(msg.message == WM_QUIT)
				break;
		}

		else
		{
		}
	}
    CleanD3D();
	return msg.wParam;
}

LRESULT CALLBACK WindowProc(HWND hWnd, UINT message, WPARAM wparam, LPARAM lparam)
{
	switch(message)
	{
	case WM_DESTROY:
		{
			PostQuitMessage(0);
			return 0;
		} break;
	}

	return DefWindowProc(hWnd, message, wparam, lparam);
}

void InitD3D(HWND hWnd)
{
	DXGI_SWAP_CHAIN_DESC scd;

	ZeroMemory(&scd, sizeof(DXGI_SWAP_CHAIN_DESC));

	scd.BufferCount = 1;
	scd.BufferDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
	scd.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;
	scd.OutputWindow = hWnd;
	scd.SampleDesc.Count = 4;
	scd.Windowed = TRUE;

	D3D11CreateDeviceAndSwapChain(NULL,
		D3D_DRIVER_TYPE_HARDWARE,
		NULL,
		NULL,
		NULL,
		NULL,
		D3D11_SDK_VERSION,
		&scd,
		&swapchain,
		&dev,
		NULL,
		&devcon);
}

void CleanD3D(void)
{
	swapchain->Release();
	dev->Release();
	devcon->Release();
}

I assume "swapchain->Release();" is where the problem is because there's a yellow arrow there (I'm a newcomer). I don't understand why though. Can someone tell me how to correct it?