title: Kotlin - Type mismatch required collection found list
body: <p>I need to append two <code>list</code> but it say me:</p>
  
  <pre><code>type mismatch required collection found list
  </code></pre>
  
  <p>It is like bellow:</p>
  
  <pre><code>val list: List&lt;Cohort&gt; = ArrayList()
  private fun fillFromDao() {
      val notesObserver: Observer&lt;ArrayList&lt;Cohort?&gt;?&gt;? =
          Observer { cohort: ArrayList&lt;Cohort?&gt;? -&gt;
                  list.toMutableList().addAll(cohort)
          }
      if (notesObserver != null) {
          otherDialogFragmentViewModel.fetchIsFree()?.observe(this, notesObserver)
          otherDialogFragmentViewModel.fetchHasCertificate()?.observe(this, notesObserver)
      }
  }
  </code></pre>