Ошибка C++ Directx: нарушение прав доступа, цепочка обмена была nullptr

Я пытаюсь изучить 3D-рисование с помощью DirectX, и я получил этот код в учебном пособии, но каждый раз, когда я пытаюсь его скомпилировать, мой компилятор выдает ошибку «Нарушение прав доступа».

заранее спасибо

   // include the basic windows header files and the Direct3D header files
    #include <windows.h>
    #include <windowsx.h>
    #include <d3d11.h>
    #include <d3dx11.h>
    #include <d3dx10.h>

    // include the Direct3D Library file
    #pragma comment (lib, "d3d11.lib")
    #pragma comment (lib, "d3dx11.lib")
    #pragma comment (lib, "d3dx10.lib")

    // global declarations
    IDXGISwapChain *swapchain;             // the pointer to the swap chain interface
    ID3D11Device *dev;                     // the pointer to our Direct3D device interface
    ID3D11DeviceContext *devcon;           // the pointer to our Direct3D device context
    ID3D11RenderTargetView *backbuffer;    // the pointer to our back buffer

    // function prototypes
    void InitD3D(HWND hWnd);    // sets up and initializes Direct3D
    void RenderFrame(void);     // renders a single frame
    void CleanD3D(void);        // closes Direct3D and releases memory

    // the WindowProc function prototype
    LRESULT CALLBACK WindowProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);


    // the entry point for any Windows program
    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 = L"WindowClass";

        RegisterClassEx(&wc);

        RECT wr = {0, 0, 800, 600};
        AdjustWindowRect(&wr, WS_OVERLAPPEDWINDOW, FALSE);

        hWnd = CreateWindowEx(NULL,
                              L"WindowClass",
                              L"Our First Direct3D Program",
                              WS_OVERLAPPEDWINDOW,
                              300,
                              300,
                              wr.right - wr.left,
                              wr.bottom - wr.top,
                              NULL,
                              NULL,
                              hInstance,
                              NULL);

        ShowWindow(hWnd, nCmdShow);

        // set up and initialize Direct3D
        InitD3D(hWnd);

        // enter the main loop:

        MSG msg;

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

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

            RenderFrame();
        }

        // clean up DirectX and COM
        CleanD3D();

        return msg.wParam;
    }


    // this is the main message handler for the program
    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);
    }


    // this function initializes and prepares Direct3D for use
    void InitD3D(HWND hWnd)
    {
        // create a struct to hold information about the swap chain
        DXGI_SWAP_CHAIN_DESC scd;

        // clear out the struct for use
        ZeroMemory(&scd, sizeof(DXGI_SWAP_CHAIN_DESC));

        // fill the swap chain description struct
        scd.BufferCount = 1;                                    // one back buffer
        scd.BufferDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;     // use 32-bit color
        scd.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;      // how swap chain is to be used
        scd.OutputWindow = hWnd;                                // the window to be used
        scd.SampleDesc.Count = 1;                               // how many multisamples
        scd.SampleDesc.Quality = 0;                             // multisample quality level
        scd.Windowed = TRUE;                                    // windowed/full-screen mode

        // create a device, device context and swap chain using the information in the scd struct
        D3D11CreateDeviceAndSwapChain(NULL,
                                      D3D_DRIVER_TYPE_HARDWARE,
                                      NULL,
                                      NULL,
                                      NULL,
                                      NULL,
                                      D3D11_SDK_VERSION,
                                      &scd,
                                      &swapchain,
                                      &dev,
                                      NULL,
                                      &devcon);


        // get the address of the back buffer
        ID3D11Texture2D *pBackBuffer;
        swapchain->GetBuffer(0, __uuidof(ID3D11Texture2D), (LPVOID*)&pBackBuffer);

        // use the back buffer address to create the render target
        dev->CreateRenderTargetView(pBackBuffer, NULL, &backbuffer);
        pBackBuffer->Release();

        // set the render target as the back buffer
        devcon->OMSetRenderTargets(1, &backbuffer, NULL);


        // Set the viewport
        D3D11_VIEWPORT viewport;
        ZeroMemory(&viewport, sizeof(D3D11_VIEWPORT));

        viewport.TopLeftX = 0;
        viewport.TopLeftY = 0;
        viewport.Width = 800;
        viewport.Height = 600;

        devcon->RSSetViewports(1, &viewport);
    }


    // this is the function used to render a single frame
    void RenderFrame(void)
    {
        // clear the back buffer to a deep blue
        devcon->ClearRenderTargetView(backbuffer, D3DXCOLOR(0.0f, 0.2f, 0.4f, 1.0f));

        // do 3D rendering on the back buffer here

        // switch the back buffer and the front buffer
        swapchain->Present(0, 0);
    }


// this is the function that cleans up Direct3D and COM
void CleanD3D(void)
{
    // close and release all existing COM objects
    swapchain->Release();
    backbuffer->Release();
    dev->Release();
    devcon->Release();
}