I would like to present an error message when in my application's constructor a file cannot be loaded. So I wrote

auto dialog = scoped!MessageDialog(this, 0, GtkMessageType.ERROR, GtkButtonsType.CLOSE, `Could not load "` ~ filename ~ `": %s`, e.msg);
dialog.run();

(I assume I don't need flags because the MessageDialog is destroyed by scope on exit of scope and dialog.run() makes it modal.)
When the dialog comes up though, it cannot be closed by clicking on OK or pressing ⏎. What could it be?

Some other things I noticed, maybe due to Gtk+3: There is no error icon and no title. There is no property for the secondary text.