title: fullCalendar is not a function and events are not rendering
body: <p>I am trying to run a very basic script that uses the interaction plugin of fullcalendar. I want the users to input events via marking the right date or time in the calendar (select). After marking for example 5th August to 9th August, a prompt would aks for the event title. User types in "vacation" and the script automatically streams the information "from... to" to the event data. But the problem is, after marking the date and typing in the event title, the event does not appear on the calendar.</p>
  
  <p>As long as the site isn't being reloaded, I also cannot mark a new event. In addition, the firefox browser console says fullCalender is not a function. The majority of what is to be found on the web about this issue is that it is most likely the library, which isn't being loaded properly, but according to the browser console its loaded properly indeed. The calendar is visible and I am able to select the date, therefore I do not see the problem. I already put hours into this....</p>
  
  <p>I already tried to change the library from local to CDN, that didn't change no thing.</p>
  
  <p>Here is the code, that I've mixed out of the documentation of fullcalendar and this code pen: <a href="https://codepen.io/JSFanatik/pen/QVvxvw" rel="nofollow noreferrer">https://codepen.io/JSFanatik/pen/QVvxvw</a></p>
  
  <pre><code>    &lt;script type="text/javascript" src='/calendar/fullcalendar/jquery-3.4.1.min.js'&gt;&lt;/script&gt;  
      &lt;script type="text/javascript" src='/calendar/fullcalendar/dist/core/main.js'&gt;&lt;/script&gt;
      &lt;script type="text/javascript" src='/calendar/fullcalendar/dist/interaction/main.js'&gt;&lt;/script&gt;
      &lt;script type="text/javascript" src='/calendar/fullcalendar/dist/daygrid/main.js'&gt;&lt;/script&gt;
      &lt;script type="text/javascript" src='/calendar/fullcalendar/dist/timegrid/main.js'&gt;&lt;/script&gt;
      &lt;script type="text/javascript" src='/calendar/fullcalendar/dist/core/locales/de.js'&gt;&lt;/script&gt;
  
      &lt;script&gt;
          document.addEventListener('DOMContentLoaded', function() {
              var calendarEl = document.getElementById('calendar');
  
              var calendar = new FullCalendar.Calendar(calendarEl, {
  
                  plugins: [ 'interaction', 'dayGrid', 'timeGrid' ],
                  locale: "de",
                  header: {
                      left: 'prev,next today',
                      center: 'title',
                      right: 'dayGridMonth,timeGridWeek,timeGridDay'
                  }, 
                  selectable: true,
                  navLinks: true,
                  defaultView: "dayGridMonth",
                  selectable: true,
                  selectHelper: false,
                  editable: true,
                  eventLimit: false, 
  
                  select: function(start, end) {
                      var title = prompt("Event Title?");
                      var eventData;
                      if (title) {
                          eventData = {
                              title: title,
                              start: start,
                              end: end
                          };        
                          $("#calendar").fullCalendar("renderEvent", eventData, true); // stick? = true
                      }
                      $("#calendar").fullCalendar("unselect");
                  },
  
                  eventRender: function(event, element) {
                      element
                      .find(".fc-content")
                      .prepend("&lt;span class='closeon material-icons'&gt;&amp;#xe5cd;&lt;/span&gt;");
                      element.find(".closeon").on("click", function() {
                          $("#calendar").fullCalendar("removeEvents", event._id);
                      });
                  },
  
                  eventClick: function(calEvent) {
                      var title = prompt("Change Title:", calEvent.title);
                      calEvent.title = title;
                      $("#calendar").fullCalendar("updateEvent", calEvent);
                  }
              });
              calendar.render();
          });
      &lt;/script&gt;
  </code></pre>
  
  <p>I hope you guys can help me with that. I have to say I don't know much about JavaScript, I just have build a website and thought this calendar would be a very useful addition. Thanks!</p>