Bug 27491 Cannot add classpath variable to project
build id : 200212020010 The buttons OK and extend... are always disabled in the add variable dialog
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/dialogfields/ListDialogField.java
/**
	 * Replace an element.
	 */		
	public void replaceElement(Object oldElement, Object newElement) throws IllegalArgumentException { 
		int idx= fElements.indexOf(oldElement);
		if (idx != -1) {
			if (oldElement.equals(newElement) || fElements.contains(newElement)) {
				return;
			}
			fElements.set(idx, newElement);
			if (fTable != null) {
				List selected= getSelectedElements();
				if (selected.remove(oldElement)) {
					selected.add(newElement);
				}
				fTable.refresh();
				selectElements(new StructuredSelection(selected));
			}
			dialogFieldChanged();
		} else {
			throw new IllegalArgumentException();
		}
	}	
	/**
	 * Adds an element at the end of the list.
	 */		
	public void addElement(Object element) {		
		if (fElements.contains(element)) {
			return;
		}