[gtk/gtk-3-24] Don't call g_object_ref on NULL, it whines.

[John Ralls <jralls src gnome org>]

commit 9c33adf118d10bbc39717b84847d92ce8be64944
Author: John Ralls <jralls ceridwen us>
Date:   Tue Apr 23 11:39:17 2019 -0700

    Don't call g_object_ref on NULL, it whines.
    
    Fixes https://gitlab.gnome.org/GNOME/gtk/issues/1822

 gdk/quartz/gdkevents-quartz.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/gdk/quartz/gdkevents-quartz.c b/gdk/quartz/gdkevents-quartz.c
index efb6961fa7..8e8f1ef183 100644
--- a/gdk/quartz/gdkevents-quartz.c
+++ b/gdk/quartz/gdkevents-quartz.c
@@ -613,7 +613,10 @@ find_toplevel_under_pointer (GdkDisplay *display,
       gint gdk_x = 0, gdk_y = 0;
       _gdk_quartz_window_nspoint_to_gdk_xy (screen_point, &gdk_x, &gdk_y);
       toplevel = _gdk_quartz_window_find_child (_gdk_root, gdk_x, gdk_y, TRUE);
-      info->toplevel_under_pointer = g_object_ref (toplevel);
+      if (toplevel)
+        info->toplevel_under_pointer = g_object_ref (toplevel);
+      else
+        info->toplevel_under_pointer = NULL;
     }
   if (toplevel)
     {