fix hot reload for babel 6

Author: yyx990803

lib/loader.js

@@ -184,7 +184,7 @@ module.exports = function (content) {
             'hotAPI.createRecord(id, module.exports)\n' +
             'module.hot.accept(' + JSON.stringify(accepted) + ', function () {\n' +
               'var newOptions = ' + (scriptString ? 'require(' + scriptString + ')\n' : 'null\n') +
-              'if (newOptions.__esModule) newOptions = newOptions.default\n' +
+              'if (newOptions && newOptions.__esModule) newOptions = newOptions.default\n' +
               'var newTemplate = ' + (templateString ? 'require(' + templateString + ')\n' : 'null\n') +
               'hotAPI.update(id, newOptions, newTemplate)\n' +
             '})\n' +