title: Add optional parameter to Android Kotlin class
body: <p>I've created an extension of <code>DialogFragment()</code>:</p>
  <pre><code>class AlertDialogFragment(context: Context, 
                            val positiveButtonText: String, 
                            val positiveButtonListener: DialogInterface.OnClickListener,
                            val negativeButtonText: String, 
                            val negativeButtonListener: DialogInterface.OnClickListener,  
                            neutralButtonText: String, 
                            neutralButtonListener: DialogInterface.OnClickListener
                            ) : DialogFragment() {
  </code></pre>
  <p>however I want the last 2 parameters to be optional.</p>
  <p>How can I achieve this?</p>
  <p>I can't set <code>neutralButtonListener: DialogInterface.OnClickListener = null</code> because <code>DialogInterface.OnClickListener</code> is a non null type.</p>