[dconf-editor] Rework GtkSettings stuff.

[Arnaud Bonatti <arnaudb src gnome org>]

commit b9eeb90a3a0a5c2333b8e601cb9e2dc1d97ae27f
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Thu Dec 7 23:43:50 2017 +0100

    Rework GtkSettings stuff.

 editor/dconf-window.vala |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/editor/dconf-window.vala b/editor/dconf-window.vala
index 73bd1e7..095a889 100644
--- a/editor/dconf-window.vala
+++ b/editor/dconf-window.vala
@@ -317,7 +317,9 @@ class DConfWindow : ApplicationWindow
     private void request_path (string full_name)
     {
 //        browser_view.set_search_mode (false);  // TODO not useful when called from bookmark
-        highcontrast = ("HighContrast" in Gtk.Settings.get_default ().gtk_theme_name);
+        Gtk.Settings? gtk_settings = Gtk.Settings.get_default ();
+        if (gtk_settings != null)
+            highcontrast = ("HighContrast" in ((!) gtk_settings).gtk_theme_name);
 
         string folder_name = SettingsModel.get_base_path (full_name);