By default, runtime error reports do not contain information on which form component caused the error. Run your server with the -Dcom.whitestein.lsps.vaadin.ui.debug=true system property to include the modeling ID of your components in the reports (if you are running your server from Designer, go to Server Connection -> Server Connection Settings, select the Connection and click Edit).

You can then use the search to find the form component with the modeling ID: go to Search > Find Form Component.