title: angular9 with primeng fullcalendar throws errors: Cannot find namespace 'FullCalendarVDom'. and 'Please import the top-level fullcalendar lib'
body: <p>I use primeng p-fullcalendar in my angular app.</p>
  <p>I get this error in command-line:
  'Cannot find namespace 'FullCalendarVDom'</p>
  <p>and in browser:
  'Please import the top-level fullcalendar lib before attempting to import a plugin.'</p>
  <p>my code:</p>
  <pre><code>import { FullCalendar } from 'primeng/fullcalendar';
  import { Calendar } from '@fullcalendar/core';
  import dayGridPlugin from '@fullcalendar/daygrid';
  import timeGridPlugin from '@fullcalendar/timegrid';
  import interactionPlugin from '@fullcalendar/interaction';
  </code></pre>
  <pre><code>@ViewChild('calendar', { static: true }) private calendar: FullCalendar;
  </code></pre>
  <p>in c'tor:</p>
  <pre><code> this.events = [{
     &quot;title&quot;: &quot;Conference&quot;,
     &quot;start&quot;: &quot;2016-01-11&quot;,
     &quot;end&quot;: &quot;2016-01-13&quot;
   }];
   this.options = {
     plugins: [dayGridPlugin, timeGridPlugin, interactionPlugin],
     defaultDate: '2017-02-01',
     header: {
       left: 'prev,next',
       center: 'title',
       right: 'month,agendaWeek,agendaDay'
     }
  
  </code></pre>
  <p>I added these imports according to preview answers, but it did not help.</p>
  <p>What I missed?</p>