Fixing first-chance exception in TodoMVC

pkg/observe/lib/src/path_observer.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of observe;
 
 // This code is inspired by ChangeSummary:
 // https://github.com/rafaelw/ChangeSummary/blob/master/change_summary.js
 // ...which underlies MDV. Since we don't need the functionality of
 // ChangeSummary, we just implement what we need for data bindings.
@@ skipping 153 matching lines @@
             _updateObservedValues(i);
             return;
           }
         }
       });
     }
   }
 }
 
 _getObjectProperty(object, property) {
+  if (object == null) {
+    return null;
+  }
+
   if (object is List && property is int) {
     if (property >= 0 && property < object.length) {
       return object[property];
     } else {
       return null;
     }
   }
 
   if (property is Symbol) {
     var mirror = reflect(object);
@@ skipping 50 matching lines @@
 
 final _spacesRegExp = new RegExp(r'\s');
 
 bool _isPathValid(String s) {
   s = s.replaceAll(_spacesRegExp, '');
 
   if (s == '') return true;
   if (s[0] == '.') return false;
   return _pathRegExp.hasMatch(s);
 }