title: Why does custom Estimator end up overriding Nothing type?
body: <p>I am implementing a custom estimator which has troubles accessing a parameter <code>ava.util.NoSuchElementException: Failed to find a default value for isInList</code>. It is defined as follows:</p>
  
  <pre><code>trait PreprocessingParams extends Params {
    final val isInList = new Param[Array[String]](this, "isInList", "list of isInList items")
  }
  </code></pre>
  
  <p>To better debug the problem I created a minimal example here <a href="https://gist.github.com/geoHeil/8dc7a6b6938a517f068e7fd6a981ed12" rel="nofollow noreferrer">https://gist.github.com/geoHeil/8dc7a6b6938a517f068e7fd6a981ed12</a> the <code>ExampleTrans</code> works just fine. However, I would rather like to include the functionality of the transformer into an estimator which performs some data cleaning as well.</p>
  
  <p>But now I face strange compile issues <code>overriding method has wrong type - expecting Nothing</code></p>
  
  <p>What is wrong with the return types of my <code>ExampleEstimator</code>?</p>