Bug 260892 [CellEditors] NPE from ColumnViewerEditor caused by showing a dialog during modify
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ColumnViewerEditor.java
ColumnViewerEditorActivationListener listener) {
		if (editorActivationListener == null) {
			editorActivationListener = new ListenerList();
		}
		editorActivationListener.add(listener);
	}
	/**
	 * Removes the given listener.
	 *
	 * @param listener
	 *            the listener to remove
	 */
	public void removeEditorActivationListener(
			ColumnViewerEditorActivationListener listener) {
		if (editorActivationListener != null) {
			editorActivationListener.remove(listener);
		}
	}
	/**
	 * Process the traverse event and opens the next available editor depending
	 * of the implemented strategy. The default implementation uses the style
	 * constants
	 * <ul>
	 * <li>{@link ColumnViewerEditor#TABBING_MOVE_TO_ROW_NEIGHBOR}</li>
	 * <li>{@link ColumnViewerEditor#TABBING_CYCLE_IN_ROW}</li>
	 * <li>{@link ColumnViewerEditor#TABBING_VERTICAL}</li>
	 * <li>{@link ColumnViewerEditor#TABBING_HORIZONTAL}</li>
	 * </ul>
	 *
	 * <p>