@Test
public void writesErrorIfNoRootNode() {

  when(rootNodeFinder.getRootNodeFromSource(null)).thenReturn(null);
  testSubject.fulfillRequest(onRequestFulfilledMock);
  verify(responseWriter, times(1))
      .writeErrorResponse(argThat((e) -> e.getMessage() == "Unable to locate root node to scan"));
  verifyOnRequestFulfilledCalled();
}