[gnome-chess] Oops, main_box should be nullable

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 529ed7207c418c9d0a126dc3c115b71548b7d9f5
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Thu Dec 24 16:09:36 2020 -0600

    Oops, main_box should be nullable
    
    Since it is our direct child, and we have to check for null in dispose.

 src/preferences-dialog.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/preferences-dialog.vala b/src/preferences-dialog.vala
index 414d9c3..a386551 100644
--- a/src/preferences-dialog.vala
+++ b/src/preferences-dialog.vala
@@ -18,7 +18,7 @@ public class PreferencesDialog : Gtk.Dialog
     private uint save_duration_timeout = 0;
 
     [GtkChild]
-    private unowned Gtk.Box main_box;
+    private unowned Gtk.Box? main_box;
     [GtkChild]
     private unowned Gtk.CheckButton show_numbering_check;
     [GtkChild]