[gtk/wip/chergert/quartz4u] macos: allow null surface

[Christian Hergert <chergert src gnome org>]

commit 06d884d3f48007eeec5a36a5271c3a80c5cd69d1
Author: Christian Hergert <chergert redhat com>
Date:   Thu Jun 18 12:35:20 2020 -0700

    macos: allow null surface

 gdk/macos/gdkmacosdevice.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gdk/macos/gdkmacosdevice.c b/gdk/macos/gdkmacosdevice.c
index 33ac9fe3be..454e2f6e9f 100644
--- a/gdk/macos/gdkmacosdevice.c
+++ b/gdk/macos/gdkmacosdevice.c
@@ -160,7 +160,7 @@ gdk_macos_device_query_state (GdkDevice        *device,
   gint y_tmp;
 
   g_assert (GDK_IS_MACOS_DEVICE (device));
-  g_assert (GDK_IS_MACOS_SURFACE (surface));
+  g_assert (!surface || GDK_IS_MACOS_SURFACE (surface));
 
   if (child_surface)
     *child_surface = surface;