Addresses #482, Tag selector does not refresh on import/delete

This addresses the delete problem. I need to talk to Simon about the import one.

chrome/content/zotero/xpcom/itemTreeView.js

@@ -207,7 +207,11 @@ Zotero.ItemTreeView.prototype.notify = function(action, type, ids)
 				else
 				{
 					var item = Zotero.Items.get(ids[i]);
-					
+					if (!item) {
+						// DEBUG: this shouldn't really happen but could if a
+						// modify comes in after a delete
+						continue;
+					}
 					if(item.isRegularItem() || !item.getSource())
 					{
 						//most likely, the note or attachment's parent was removed.

chrome/content/zotero/xpcom/notifier.js

@@ -267,6 +267,9 @@ Zotero.Notifier = new function(){
 	}
 	
 	
+	/*
+	 * These should probably no longer be used now that we have event queuing
+	 */
 	function disable(){
 		Zotero.debug('Disabling Notifier notifications');
 		_disabled = true;