[gnome-photos] Fix crash when reading scale factor after window has been destroyed

[Debarshi Ray <debarshir src gnome org>]

commit c250ff800d218a81538c22e29732892e4d4e66d8
Author: Debarshi Ray <debarshir gnome org>
Date:   Mon Nov 10 15:07:36 2014 +0100

    Fix crash when reading scale factor after window has been destroyed
    
    Ideally we should cancel all pending operations when the main window
    has been destroyed, but that will require a more intrusive change. I am
    uncomfortable making such a change in a stable bug-fix release unless
    I can reproduce this scenario to ensure that it does not regress
    anything.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=738527

 src/photos-application.c |   11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)
---
diff --git a/src/photos-application.c b/src/photos-application.c
index bd6c010..8f0860d 100644
--- a/src/photos-application.c
+++ b/src/photos-application.c
@@ -1085,7 +1085,7 @@ gint
 photos_application_get_scale_factor (PhotosApplication *self)
 {
   GList *windows;
-  gint scale_factor;
+  gint ret_val = 1;
 
   /* We do not use priv->main_window to allow widgets to use this
    * method while they are being constructed. The widget hierarchy is
@@ -1093,6 +1093,11 @@ photos_application_get_scale_factor (PhotosApplication *self)
    * priv->main_window is NULL.
    */
   windows = gtk_application_get_windows (GTK_APPLICATION (self));
-  scale_factor = gtk_widget_get_scale_factor (GTK_WIDGET (windows->data));
-  return scale_factor;
+  if (windows == NULL)
+    goto out;
+
+  ret_val = gtk_widget_get_scale_factor (GTK_WIDGET (windows->data));
+
+ out:
+  return ret_val;
 }