[four-in-a-row] Fix a bug.

[Arnaud B. <arnaudb src gnome org>]

commit 6f27cf1be6dde3c2472fb799f93c004e88b0e7c7
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Fri Dec 20 19:09:33 2019 +0100

    Fix a bug.

 src/game-window.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/game-window.vala b/src/game-window.vala
index 3510bee..44ac972 100644
--- a/src/game-window.vala
+++ b/src/game-window.vala
@@ -277,7 +277,8 @@ private class GameWindow : AdaptativeWindow, AdaptativeWidget
         {
             if (game_widget_1 != null)
                 ((!) game_widget_1).show ();
-            if (stack.get_visible_child_name () == "start-box")
+            string? panel_name = stack.get_visible_child_name ();
+            if (panel_name != null && (!) panel_name == "start-box")
                 headerbar.set_title (program_name);
             else
                 headerbar.set_title (last_subtitle);