Bug 130854 [CellEditors] JFace TableViewer ignoring ICellEditor validator state
Using a validator on a TextCellEditor and entering invalid text into a cell results in ICellModifier.modify() being called with a null value, rather than an error being reported and the value being discarded. TableEditorImpl.saveEditorValue() *does actually check for editor validity*, but in the case of invalid values does nothing but no-op with comment "///Do what ???" and then calls cellEditor.getValue() which, for TextCellEditor's anyway, always returns null if its validator has invalidated the current value.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ColumnViewerEditor.java
* set the layout data for the editor
	 *
	 * @param layoutData
	 *            the layout data used when editor is displayed
	 */
	protected abstract void setLayoutData(CellEditor.LayoutData layoutData);
	/**
	 * @param focusCell
	 *            updates the cell with the current input focus
	 * @param event
	 *            the event requesting to update the focusCell
	 */
	protected abstract void updateFocusCell(ViewerCell focusCell,
			ColumnViewerEditorActivationEvent event);
	/**
	 * @return the cell currently holding the focus if no cell has the focus or
	 *         the viewer implementation doesn't support <code>null</code> is
	 *         returned
	 *
	 */
	public ViewerCell getFocusCell() {
		return null;
	}
	/**
	 * @return the viewer working for
	 */
	protected ColumnViewer getViewer() {
		return viewer;
	}
}