An error occurs when trying to create a text field with the type java.lang.Long (as per https://docs.magnolia-cms.com/product-docs/Templating/Dialog-definition/Field-definition/List-of-fields/Text-field.html). If you use that Java class, you also need to use com.vaadin.data.converter.StringToLongConverter as converterClass (https://jira.magnolia-cms.com/browse/MGNLUI-6355).

Update the Text field page with this info.