title: AttributeError: 'NoneType' object has no attribute 'index' using RASA
body: <p>i try to use RASA to build a sample bot, but i got ths error message,</p>
  
  <pre><code>Bot loaded. Type a message and press enter (use '/stop' to exit):
  sad
  Exception in thread Thread-2:
  Traceback (most recent call last):
    File "C:\python36\lib\threading.py", line 916, in _bootstrap_inner
      self.run()
    File "C:\python36\lib\threading.py", line 864, in run
      self._target(*self._args, **self._kwargs)
    File "C:\python36\lib\site-packages\rasa_core\channels\channel.py", line 291, in on_message_wrapper
      on_new_message(message)
    File "C:\python36\lib\site-packages\rasa_core\agent.py", line 316, in handle_message
      return processor.handle_message(message)
    File "C:\python36\lib\site-packages\rasa_core\processor.py", line 86, in handle_message
      self._predict_and_execute_next_action(message, tracker)
    File "C:\python36\lib\site-packages\rasa_core\processor.py", line 305, in _predict_and_execute_next_action
      action, policy, confidence = self.predict_next_action(tracker)
    File "C:\python36\lib\site-packages\rasa_core\processor.py", line 168, in predict_next_action
      probabilities, policy = self._get_next_action_probabilities(tracker)
    File "C:\python36\lib\site-packages\rasa_core\processor.py", line 478, in _get_next_action_probabilities
      tracker, self.domain)
    File "C:\python36\lib\site-packages\rasa_core\policies\ensemble.py", line 288, in probabilities_using_best_policy
      if (result.index(max_confidence) ==
  AttributeError: 'NoneType' object has no attribute 'index'
  </code></pre>
  
  <p>i use :</p>
  
  <pre><code>PYTHON  3.6.0
  RASA CORE   0.12.0
  RASA NLU    0.13.0
  TENSORFLOW  1.10.0
  PROTOBUF    3.6.0
  </code></pre>
  
  <p>ho it possible to solve this ?
  Thank you</p>