Bullet-proofing export scenarios: null attachments, no msgreceiver

FREEBIE

js/background.js

@@ -334,7 +334,7 @@
                 Whisper.events.trigger('reconnectTimer');
             } else {
                 console.log('offline');
-                messageReceiver.close();
+                if (messageReceiver) { messageReceiver.close(); }
                 window.addEventListener('online', init);
             }
             return;

js/backup.js

@@ -369,7 +369,7 @@
             var jsonString = JSON.stringify(stringify(message));
             stream.write(jsonString);
 
-            if (attachments.length) {
+            if (attachments && attachments.length) {
               var process = function() {
                 return writeAttachments(dir, name, messageId, attachments);
               };