title: Not Fitted Error when using Sklearn's graphviz
body: <p>when I am trying to export a random forest graph using the following command:</p>
  
  <pre><code>tree.export_graphviz(rnd_clf, out_file = None, feature_names = X_test[::1])
  </code></pre>
  
  <p>I receive the following error:</p>
  
  <pre><code>NotFittedError: This RandomForestClassifier instance is not fitted yet. 
  Call 'fit' with appropriate arguments before using this method.
  </code></pre>
  
  <p>What I don't understand is why it keeps telling me this, even though I have fitted the random forest classifier using:</p>
  
  <pre><code>rnd_clf = RandomForestClassifier(  
               n_estimators=120,
               criterion='gini',
               max_features= None, 
               max_depth = 14 )
  
  rnd_clf.fit(X_train, y_train)
  </code></pre>
  
  <p>and it works perfectly fine.</p>