Fix null-ref error

Fixed "TypeError: this.refs.btn is undefined" caused by inline-ref-callback.
See https://reactjs.org/docs/refs-and-the-dom.html#caveats

patterns/19.async-nature-of-setState.md

@@ -25,6 +25,11 @@ make sure the code that follows has access to the latest information available.
 
 ```javascript
 class TestComponent extends React.Component {
+  constructor(...args) {
+    super(...args);
+    this._saveButtonRef = (btn => { this._btnRef = btn });
+  }
+
   getInitialState() {
     return {
       dollars: 10
@@ -41,7 +46,7 @@ class TestComponent extends React.Component {
     // effects on how state mutates
     //
     // Check the list here - https://facebook.github.io/react/docs/events.html
-    this.refs.btn.addEventListener('mouseleave', this._onMouseLeaveHandler);
+    this._btnRef.addEventListener('mouseleave', this._onMouseLeaveHandler);
 
     // Add JS timeout
     //
@@ -59,7 +64,7 @@ class TestComponent extends React.Component {
 
     return (
        <button
-         ref={(btn) => this.btn = btn}
+         ref={this._saveButtonRef}
          onClick={this._onClickHandler}>
          'Click me'
       </button>