[gtk+] gtkapplication: Mark gtk_application_get_active_window() as nullable

[Matthias Clasen <matthiasc src gnome org>]

commit 4b1853b32fff3202daa32276191dc2214aef5bf3
Author: Arun Raghavan <arun arunraghavan net>
Date:   Thu Jul 13 11:25:11 2017 +0530

    gtkapplication: Mark gtk_application_get_active_window() as nullable
    
    https://bugzilla.gnome.org/show_bug.cgi?id=784888

 gtk/gtkapplication.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/gtk/gtkapplication.c b/gtk/gtkapplication.c
index ff70808..da67121 100644
--- a/gtk/gtkapplication.c
+++ b/gtk/gtkapplication.c
@@ -860,7 +860,8 @@ gtk_application_get_window_by_id (GtkApplication *application,
  * if another application has it — this is just the most
  * recently-focused window within this application.
  *
- * Returns: (transfer none): the active window
+ * Returns: (transfer none) (nullable): the active window, or %NULL if
+ *   there isn't one.
  *
  * Since: 3.6
  **/