Calling Gtk.MessageDialog with an empty string for message_format like this:

var msg = new Gtk.MessageDialog (parent, Gtk.DialogFlags.MODAL,
        Gtk.MessageType.WARNING, Gtk.ButtonsType.OK, "");

The generated code will produce a compilation warning:

src/application.c:1064:118: warning: zero-length gnu_printf format string [-Wformat-zero-length]
  _tmp3_ = (GtkMessageDialog*) gtk_message_dialog_new (_tmp2_, GTK_DIALOG_MODAL, GTK_MESSAGE_WARNING, GTK_BUTTONS_OK, "");


According to the documentation gtk_message_dialog_new could be called with null for message_format, so trying this:

var msg = new Gtk.MessageDialog (parent, Gtk.DialogFlags.MODAL,
        Gtk.MessageType.WARNING, Gtk.ButtonsType.OK, null);

But now the generated code will instead pass "%s" as message_format with a single parameter null, resulting in the following warnings:

src/application.c:1064:2: warning: reading through null pointer (argument 6) [-Wformat=]
  _tmp3_ = (GtkMessageDialog*) gtk_message_dialog_new (_tmp2_, GTK_DIALOG_MODAL, GTK_MESSAGE_WARNING, GTK_BUTTONS_OK, "%s", NULL);
  ^~~~~~
src/application.c:1064:120: warning: format '%s' expects argument of type 'char *', but argument 6 has type 'void *' [-Wformat=]
  _tmp3_ = (GtkMessageDialog*) gtk_message_dialog_new (_tmp2_, GTK_DIALOG_MODAL, GTK_MESSAGE_WARNING, GTK_BUTTONS_OK, "%s", NULL);