I've wrote a simple application (don't know if I've done this right):

import gtk.Application;
import gtk.ApplicationWindow;

int main(string[] args) {
    auto app = new Application("org.gtk.d.demo", ApplicationFlags.FLAGS_NONE);
    auto appWindow = new ApplicationWindow(app);
    app.addWindow(appWindow);
    appWindow.showAll();
    return app.run(args);
}

When I try to run this I get this:

Running ./target/test 

(process:3686242): Gtk-CRITICAL **: 02:35:18.273: _gtk_style_provider_private_get_settings: assertion 'GTK_IS_STYLE_PROVIDER_PRIVATE (provider)' failed

(process:3686242): Gtk-CRITICAL **: 02:35:18.273: _gtk_style_provider_private_get_settings: assertion 'GTK_IS_STYLE_PROVIDER_PRIVATE (provider)' failed

(process:3686242): Gtk-CRITICAL **: 02:35:18.273: _gtk_style_provider_private_get_settings: assertion 'GTK_IS_STYLE_PROVIDER_PRIVATE (provider)' failed
Program exited with code -11

What is the problem?