TYPO3 Core

• Go to the form module in the backend
• Click the "Copy" button on any form element
• Fill out the wizard and click both "next" buttons

You should now see the following exception for a missing savePath:

(1/1) #1298012500 TYPO3\CMS\Extbase\Mvc\Controller\Exception\RequiredArgumentMissingException
Required argument "savePath" is not set for TYPO3\CMS\Form\Controller\FormManagerController->duplicate.

10 introduced the MultiStepWizard for the copying behaviour in the file typo3/sysext/form/Resources/Public/JavaScript/Backend/FormManager/ViewModel.js

There, in Wizard Step 2, a click event is registered for the next button, that saves the value of a select element in the savePath property:

nextButton.on('click', function(e) {
            Icons.getIcon('spinner-circle', Icons.sizes.default, null, null).then(function(markup) {
              MultiStepWizard.set('confirmationDuplicateFormName', that.data('formName'));
              MultiStepWizard.set('savePath', $(getDomElementIdentifier('duplicateFormSavePath') + ' option:selected', modal).val());

              if (folders.length > 1) {
                MultiStepWizard.set('confirmationDuplicateFormSavePath', $(getDomElementIdentifier('duplicateFormSavePath') + ' option:selected', modal).text());
              } else {
                MultiStepWizard.set('confirmationDuplicateFormSavePath', folders[0]['label']);
              }

              slide.html($('<div />', {class: 'text-center'}).append(markup));
            });
          });

If you now click "next" and are directed to Wizard Step 3, this click event remains on the new "next" button. So clicking the "next" button of step 3 will now trigger the click event again and this time savePath will be null, because the "duplicateFormSavePath" select is not present in step 3.