Hello,

After updating to React 15.0.1, we are getting the following error:

```
Uncaught TypeError: Cannot read property 'addEventListener' of null
```

![image](https://cloud.githubusercontent.com/assets/4996164/14602577/5bb4e9f2-0571-11e6-9aa1-98de49f50d76.png)

* @return {?object}
   * @private
   */
  _maskContext: function(context) {
    var Component = this._currentElement.type;
    var contextTypes = Component.contextTypes;
    if (!contextTypes) {
      return emptyObject;
    }
    var maskedContext = {};
    for (var contextName in contextTypes) {
      maskedContext[contextName] = context[contextName];
    }
    return maskedContext;
  },
  /**
   * Filters the context object to only contain keys specified in
   * `contextTypes`, and asserts that they are valid.
   *
   * @param {object} context
   * @return {?object}
   * @private
   */
  _processContext: function(context) {
    var maskedContext = this._maskContext(context);
    if (__DEV__) {
      var Component = this._currentElement.type;
      if (Component.contextTypes) {
        this._checkPropTypes(
          Component.contextTypes,