[geary/wip/765516-gtk-widget-conversation-viewer] Fix a critical error if ConversationListBox is destroyed twice.

[Michael Gratton <mjog src gnome org>]

commit 02a89d9e80ec71639f35f9ee0925f9903dc9e236
Author: Michael James Gratton <mike vee net>
Date:   Mon Sep 26 17:37:31 2016 +1000

    Fix a critical error if ConversationListBox is destroyed twice.

 .../conversation-viewer/conversation-listbox.vala  |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/src/client/conversation-viewer/conversation-listbox.vala 
b/src/client/conversation-viewer/conversation-listbox.vala
index a077909..5f6f538 100644
--- a/src/client/conversation-viewer/conversation-listbox.vala
+++ b/src/client/conversation-viewer/conversation-listbox.vala
@@ -296,16 +296,18 @@ public class ConversationListBox : Gtk.ListBox {
                     show_loading();
                 }
                 this.loading_timeout_id = 0;
-                return false;
+                return Source.REMOVE;
             });
     }
 
     public override void destroy() {
         if (this.loading_timeout_id != 0) {
             Source.remove(this.loading_timeout_id);
+            // Clear in case this is called twice
+            this.loading_timeout_id = 0;
         }
         this.cancellable.cancel();
-        this.id_to_row.clear();
+        this.email_rows.clear();
         base.destroy();
     }