LRESULT CALLBACK 
mainWindowProc(HWND window, 
               UINT message, 
	       WPARAM wParam, 
	       LPARAM lParam)
{
    LONG_PTR userdata = GetWindowLongPtrA(window, GWLP_USERDATA);
    State *state = (State *)userdata; // <- Null pointer given right here...

    switch(message)
    {
        case WM_SIZE:
        {
            int width = LOWORD(lParam);
    	    int height = HIWORD(lParam);

            RECT windowRectangle;
            GetClientRect(window, &windowRectangle);
            u32 drawingWindowWidth = windowRectangle.right - windowRectangle.left;
            u32 drawingWindowHeight = windowRectangle.bottom - windowRectangle.top;

            windowResize(state, window, (u32)wParam, drawingWindowWidth, drawingWindowHeight); // <- Null pointer passed right here
        }break;

        ...
    }
    ...
}

int CALLBACK 
WinMain(HINSTANCE instance, 
        HINSTANCE prevInstance, 
	LPSTR commandLine, 
	int showCommand)
{
    // blah blah creating window classes
    
    State state = {};
    windowLoad(&state, commandLine);
    HWND window = CreateTheDamnWindowFunction([insertparamshere]);

    SetWindowLongPtrA(window, GWLP_USERDATA, (LONG_PTR)&state);
    
    .... (later on in the code)
    while(PeekMessage(&message, 0, 0, 0, PM_REMOVE))
    {
        TranslateMessage(&message);
	DispatchMessageA(&message);
    }

    ....
}


(in another file)

void
// Todo: Do we need to pass the HWND struct into here?
windowResize(State *state, HWND window, u32 wParam, int width, int height)
{
    // NOTE: wParam
    // SIZE_MAXHIDE   := 4 | Message is sent to all pop-up windows when some other window is maximized
    // SIZE_MAXSHOW   := 3 | Message is sent to all pop up windows when some other window has been restored to its former size
    // SIZE_MAXIMIZED := 2 | The window has been maximized
    // SIZE_MINIMIZED := 1 | The window has been minimized
    // SIZE_RESTORED  := 0 | The window has been resized, but neither the SIZE_MINIMIZED nor SIZE_MAXIMIZED value applies

    state->windowInfo.width = width; // <- WHABAM U TRIED TO USE A NULL POINTER, PREPARE FOR DESTRUCTION *Crash, ded program*
    state->windowInfo.height = height;
    bitmapResize(&state->backBuffer, width, height);
}

    your_type *This;
    if(Message == WM_CREATE)
    {
        CREATESTRUCT *CreateStruct = (CREATESTRUCT *)LParam;
        This = (your_type *)CreateStruct->lpCreateParams;
        SetWindowLongPtr(Window, GWLP_USERDATA, (LONG_PTR)This);
    }
    else
    {
        This = (your_type *)GetWindowLongPtr(Window, GWLP_USERDATA);
    }