How to use Pango::Context?

Hello
I try to list the awailable font-families.
If I open and close multiple windows ther is a critical error:
GLib-GObject-CRITICAL **: 15:22:32.722: g_object_unref: assertion
'G_IS_OBJECT (object)' failed

This is the code, the error ocours even without the uncomented code:

#include <gtkmm/application.h>
#include <gtkmm/window.h>
#include <gtkmm/textview.h>

class Window: public Gtk::Window
{
public:
	Window()
	{
		this->show();
		this->set_child(this->text);

		auto context = this->text.create_pango_context();
		auto fontMap = context->get_font_map();
//		auto fontList = fontMap->list_families();
//		auto buffer = this->text.get_buffer();
//		auto iter = buffer->get_iter_at_offset(0);
//		for(const auto& fam : fontList)
//		{
//			iter = buffer->insert(iter,
//				fam->get_name() + '\n');
//		}
//	}

private:
	Gtk::TextView text;
};

auto main() -> int
{
	std::vector<Window> windows;
	auto app = Gtk::Application::create();

	app->signal_activate().connect([&]
	{
		auto& w0 = windows.emplace_back();
		app->add_window(w0);
		auto& w1 = windows.emplace_back();
		app->add_window(w1);
	});

	return app->run();
}