'BeginDraw' in conjunction with D2D1 causes an error

I'm trying to clear the background of a window created with CreateWindowEx with the code separated into separate classes, with the use of D2D1. Here's the code.

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam) {
    switch (msg) {
    case WM_PAINT:
        graphics->BeginDraw();
        graphics->EndDraw();
        break;

    case WM_CREATE:
        window->onCreate();
        break;

    case WM_DESTROY:
        window->onDestroy();
        ::PostQuitMessage(0);
        break;

    default:
        return ::DefWindowProc(hwnd, msg, wparam, lparam);
    }

    return NULL;
}

^ Window.cpp

bool Graphics::init(HWND windowHandle) {
    HRESULT hRes = D2D1CreateFactory(
        D2D1_FACTORY_TYPE_SINGLE_THREADED,
        &factory
    );

    if (hRes != S_OK) return false;

    RECT wCS;
    GetClientRect(windowHandle, &wCS);

    hRes = factory->CreateHwndRenderTarget(
        D2D1::RenderTargetProperties(),
        D2D1::HwndRenderTargetProperties(windowHandle, D2D1::SizeU(wCS.right, wCS.bottom)),
        &renderTarget
    );

    if (hRes != S_OK) return -1;
    return true;
}

void Graphics::ClearScreen(float r, float g, float b) {
    renderTarget->Clear(D2D1::ColorF(r, g, b));
}

^ Graphics.cpp

void BeginDraw() { renderTarget->BeginDraw(); }
void EndDraw() { renderTarget->EndDraw(); }

^ Graphics.h

My error occurs as soon as the window opens, just before it crashes. The BeginDraw() is responsible for it, as removing it from WM_PAINT resolves the issue, but that is not a viable option.

The error:

Exception thrown: read access violation. this was nullptr.

It is specified to be on the line void BeginDraw() { ... } in Graphics.h.

Notable things (Not important):

I've been trying to find a solution to this over the past several days, but I don't know what most of the code I read means, so I apologise for my lack of knowledge.

I presume the issue is BeginDraw() is being called before the graphics have initialised, but I'm not sure how to prevent that.

Thank you for any suggestions what so ever.

The problem is the graphics object being used for BeginDraw is a different object to the one which was initialised. It needs to be the same object.