title: FragmentDirections was not generate by Safe Args plugin
body: <p>When I trying to use Safe Args plugin to jump from one fragment to another, the compiler shows "Unresolved reference: VideoFragmentDirections".
  I have already set the classpath and dependency for safe args and VideoFragmentArgs was generated correctly.
  kotlin_version = '1.3.50'
  navigation-safe-args-gradle-plugin:2.1.0
  In my xml</p>
  
  <pre><code>&lt;fragment
          android:id="@+id/nav_home"
          android:name="example.ui.home.HomeFragment"
          android:label="@string/menu_home"
          tools:layout="@layout/fragment_home" &gt;
          &lt;action
              android:id="@+id/action_nav_home_to_videoFragment"
              app:destination="@id/videoFragment"
              app:popUpTo="@+id/nav_home"/&gt;
      &lt;/fragment&gt;
  &lt;fragment
          android:id="@+id/videoFragment"
          android:name="example.ui.videoui.VideoFragment"
          android:label="VideoFragment" &gt;
          &lt;argument
              android:name="id"
              app:argType="long"
              android:defaultValue="0L" /&gt;
      &lt;/fragment&gt;
  </code></pre>
  
  <p>And in HomeFragment.ky</p>
  
  <pre><code>override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
          super.onViewCreated(view, savedInstanceState)
          val videoViewModel = ViewModelProviders
              .of(this)
              .get(HomeViewModel::class.java)
          val adapter = video_list.adapter!! as PlaylistAdapter
          videoViewModel.videos.observe(this, Observer&lt;List&lt;Video&gt;&gt; {
              adapter.submitList(it)
              adapter.onItemClickListener = View.OnClickListener { v -&gt;
  
                  val viewHolder = v.tag as RecyclerView.ViewHolder
                  val position = viewHolder.adapterPosition
                  val id = it[position].id
                  val action = VideoFragmentDirections.actionhHomeToVideo(id)
                  view.findNavController().navigate(action)
              }
          })
  }
  </code></pre>
  
  <p>Also, I had tried to clean and rebuild the project, it still doesn't work.</p>