title: ReactDOM.findDOMNode with getAttribute("") returns null
body: <p>I am defining tooltip as </p>
  
  <pre><code>&lt;MACDTooltip ref="MACDTooltip" forChart={chartId} forDataSeries={dataSeriesId} key={`${chartId}-${dataSeriesId}`} calculator={macdCalculator}
                  onClick={logger.bind(null, { chartId, dataSeriesId }, options)} origin={[-48, 15]}/&gt; 
  </code></pre>
  
  <p>but in <code>componentDidMount()</code> method </p>
  
  <pre><code>ReactDOM.findDOMNode(this.refs.MACDTooltip).getAttribute("transform")
  </code></pre>
  
  <p>return <code>null</code></p>
  
  <pre><code>ReactDOM.findDOMNode(this.refs.MACDTooltip)
  </code></pre>
  
  <p>returns </p>
  
  <pre><code> &lt;g&gt;&lt;g class="react-stockcharts-toottip" transform="translate(-48, 15)"...&lt;/g&gt;&lt;/g&gt;
  </code></pre>
  
  <p>and </p>
  
  <p>ReactDOM.findDOMNode(this.refs.MACDTooltip).innerHTML</p>
  
  <p>return <code>"&lt;g class="react-stockcharts-toottip" transform="translate(-48, 15)"&gt;&lt;/g&gt;</code></p>
  
  <p>How do i use <code>ReactDOM.findDOMNode</code> to get proper value</p>