title: Parsley Validation throwing: Validator `undefined` does not handle multiple values
body: <p>I have a form in a modal I'm trying to manually POST using ajax after it has been successfully validated by Parsley. When the modal is opened, I first initialise Parsley on the form with:</p>
  
  <pre><code>modalLaunchers.createTaskFormValidation = $(document.getElementById('create-task-form')).parsley(parsleyOptions);
  </code></pre>
  
  <p>When the form is then submitted, I have the following code:</p>
  
  <pre><code>  modalLaunchers.createTaskFormValidation.validate();
    if (modalLaunchers.createTaskFormValidation.isValid()) {
           // do ajax POST
      }
  </code></pre>
  
  <p>If there are incorrect fields, these are correctly highlighted. However, if those fields are then corrected and the form resubmitted, I get the following error:</p>
  
  <pre><code>Uncaught Validator `undefined` does not handle multiple values
  </code></pre>
  
  <p>Both validate() and isValid() throw that error. </p>
  
  <p>Obviously I want the form to be re-validated when submit is pressed again and if it is valid, continue to the POST. However, because that error is thrown it doesn't proceed any further. My Parsely options are below:</p>
  
  <pre><code>const parsleyOptions = {
      // errorsWrapper: '',
      errorTemplate: '&lt;span class="error-msg"&gt;&lt;/span&gt;',
      // successClass: 'has-success',
      errorClass: 'has-error',
      classHandler: (el) =&gt; {
          console.log(el.$element.closest('div.form-group'));
          return el.$element.closest('div.form-group');   
      },
      excluded: 'input.select2-search__field',
  };
  </code></pre>
  
  <p>Any suggestions? Thanks a lot</p>