Call to undefined function template_preprocess_webform_element_managed_file()

Adding a dropzone element and trying to access the webform submissions page returns a fatal error.

"Error: Call to undefined function template_preprocess_webform_element_managed_file() in template_preprocess_webform_element_dropzonejs() (line 31 of /var/www/xxx/yyyy/web/modules/contrib/dropzonejs_webform/webform_dropzonejs.module).