New Test
open | /run/com.vaadin.tests.components.UnknownComponentConnectorTest?restartApplication |

assertText

vaadin=runcomvaadintestscomponentsUnknownComponentConnectorTest::/VVerticalLayout[0]/VVerticalLayout[0]/VUnknownComponent[0]/domChild[0]/domChild[0]/domChild[0]/domChild[0]

Widgetset does not contain implementation for com.vaadin.tests.components.UnknownComponentConnectorTest.ComponentWithoutConnector. Check its component connector's @Connect mapping, widgetsets GWT module description file and re-compile your widgetset. In case you have downloaded a vaadin add-on package, you might want to refer to add-on instructions.