### Question:
RelationColumn extends DMNSimpleGridColumn<RelationGrid, String> implements HasSingletonDOMElementResource { @Override public void destroyResources() { super.destroyResources(); factory.destroyResources(); } RelationColumn(final HeaderMetaData headerMetaData,
                          final TextAreaSingletonDOMElementFactory factory,
                          final double width,
                          final RelationGrid gridWidget); @Override void edit(final GridCell<String> cell,
                     final GridBodyCellRenderContext context,
                     final Consumer<GridCellValue<String>> callback); @Override void flush(); @Override void destroyResources(); @Override void setWidth(final double width);  }### Answer:
@Test public void testHeaderDOMElementsAreDestroyed() { final MockHasDOMElementResourcesHeaderMetaData mockHeaderMetaData = mock(MockHasDOMElementResourcesHeaderMetaData.class); column.getHeaderMetaData().add(mockHeaderMetaData); column.destroyResources(); verify(mockHeaderMetaData).destroyResources(); }