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

@@ -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");