Why is this GUI code crashing on startup? (Windows/Bitwig+Reaper)

[Majosto]

I'm trying to setup a window class to draw the GUI, but what works perfectly fine as a standalone app crashes when placed inside the plugin. Any ideas what's causing the problem?

Here's the code I'm working with:

#include <vector>
template <class derived>
class BaseWindow
{
protected:
	HWND hWnd;
	virtual PCWSTR className() const = 0;
	virtual LRESULT handleMessage(UINT uMsg, WPARAM wParam, LPARAM lParam) = 0;

public:
	BaseWindow() : hWnd(NULL) {}

	BOOL createWindow(
		PCWSTR lpWindowName,
		DWORD dstyle,
		HCURSOR hCursor = LoadCursor(NULL, IDC_ARROW),
		DWORD dwExStyle = 0,
		int x = CW_USEDEFAULT,
		int y = CW_USEDEFAULT,
		int nWidth = CW_USEDEFAULT,
		int nHeight = CW_USEDEFAULT,
		HWND hWndParent = 0,
		HMENU hMenu = 0)
	{

		WNDCLASS wc{};
		wc.lpszClassName = className();
		wc.hInstance = GetModuleHandle(NULL);
		wc.lpfnWndProc = derived::windowProc;
		wc.hCursor = hCursor;

		RegisterClass(&wc);

		hWnd = CreateWindowExW(
			dwExStyle,
			className(),
			lpWindowName,
			dstyle,
			x,
			y,
			nWidth,
			nHeight,
			hWndParent,
			hMenu,
			GetModuleHandle(NULL),
			this
		);

		return (hWnd ? TRUE : FALSE);
	}

	HWND Window() const { return hWnd; }


	static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
	{
		derived* pThis{ NULL };
		if (uMsg == WM_NCCREATE)
		{
			CREATESTRUCT* pCreate = (CREATESTRUCT*)lParam;
			pThis = (derived*)pCreate->lpCreateParams;
			SetWindowLongPtr(hWnd, GWLP_USERDATA, (LONG_PTR)pThis);

			pThis->hWnd = hWnd;
		}
		else
		{
			pThis = (derived*)GetWindowLongPtr(hWnd, GWLP_USERDATA);
		}

		if (pThis)
			return pThis->handleMessage(uMsg, wParam, lParam);
		else
			return DefWindowProc(hWnd, uMsg, wParam, lParam);
	}
};

class HarmonicsWindow : public BaseWindow<HarmonicsWindow>
{

public:

	LRESULT handleMessage(UINT uMsg, WPARAM wParam, LPARAM lParam)
	{
		return DefWindowProc(hWnd, uMsg, wParam, lParam);
	}
	PCWSTR className() const { return L"HarmonicsWindow"; }
};

struct my_plugin_t {
public:
    clap_plugin_t                   plugin;
    const clap_host_t* host;
    const clap_host_latency_t* host_latency;
    const clap_host_log_t* host_log;
    const clap_host_thread_check_t* host_thread_check;
    const clap_host_state_t* host_state;
    const clap_host_timer_support_t* host_timer;
    clap_id timerID;
    uint32_t latency;
    HarmonicsWindow window{};
};

bool my_gui_create(const clap_plugin_t* plugin, const char* api, bool is_floating)
{
    HRESULT hr = ((my_plugin_t*)plugin->plugin_data)->window.createWindow(
        L"Window",
        WS_CHILD,
        LoadCursor(NULL, IDC_ARROW),
        0,
        0, 0,
        GUI_WIDTH, GUI_HEIGHT,
        NULL,
        0);
    if (SUCCEEDED(hr))
        return true;
    else
        return false;

}

bool my_gui_set_parent(const clap_plugin_t* plugin, const clap_window_t* window)
{
    my_plugin_t* myPlugin = ((my_plugin_t*)plugin->plugin_data);
    SetParent(myPlugin->window.Window(), (HWND)window->win32);
    ShowWindow(myPlugin->window.Window(), SW_SHOW);

    return true;
}