[gnome-todo] manager: avoid several criticals due empty list

[Victor Toso de Carvalho <victortoso src gnome org>]

commit 8d15acb5250b32c93e4669e932572e8d8d1dea3c
Author: Victor Toso <me victortoso com>
Date:   Thu Oct 27 17:38:23 2016 +0200

    manager: avoid several criticals due empty list
    
    GLib-GObject-WARNING **: invalid (NULL) pointer instance
    GLib-GObject-CRITICAL **: g_signal_handlers_disconnect_matched:
    assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed
    
    Signed-off-by: Victor Toso <victortoso redhat com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=773590

 src/engine/gtd-manager.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/engine/gtd-manager.c b/src/engine/gtd-manager.c
index 125fcfa..509469e 100644
--- a/src/engine/gtd-manager.c
+++ b/src/engine/gtd-manager.c
@@ -440,6 +440,9 @@ gtd_manager__list_removed (GtdProvider *provider,
 {
   GtdManagerPrivate *priv = gtd_manager_get_instance_private (self);
 
+  if (!list)
+      return;
+
   priv->tasklists = g_list_remove (priv->tasklists, list);
 
   g_signal_handlers_disconnect_by_func (list,