[gnome-control-center] display: Fix a crash when opening the panel with the lid closed

[Rui Matos <rtcm src gnome org>]

commit e5a1afee9ce6e846bb171cc1341084c3d5d61ac2
Author: Rui Matos <tiagomatos gmail com>
Date:   Tue Aug 22 19:23:46 2017 +0200

    display: Fix a crash when opening the panel with the lid closed
    
    The configuration manager might not be ready yet when we're called
    from the lid state monitor handler. Don't do anything in that case.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=786080

 panels/display/cc-display-panel.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/panels/display/cc-display-panel.c b/panels/display/cc-display-panel.c
index 71f5bf0..b3883fe 100644
--- a/panels/display/cc-display-panel.c
+++ b/panels/display/cc-display-panel.c
@@ -2179,6 +2179,9 @@ on_screen_changed (CcDisplayPanel *panel)
   GList *outputs, *l;
   GtkWidget *main_widget;
 
+  if (!priv->manager)
+    return;
+
   reset_titlebar (panel);
 
   main_widget = gtk_stack_get_child_by_name (GTK_STACK (priv->stack), "main");