Bug 75711 AstProvider should not create AST if requested by SelectionListenerWithASTManager
3.1 M2 If SelectionListenerWithASTManager waits for an AST and the editor closes we should not create an AST for that one but return null instead.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/JavaEditor.java
* The internal shell activation listener for updating occurrences.
	 * @since 3.0
	 */
	private ActivationListener fActivationListener= new ActivationListener();
	private ISelectionListenerWithAST fPostSelectionListenerWithAST;
	private OccurrencesFinderJob fOccurrencesFinderJob;
	
	private OccurrencesFinderJobCanceler fOccurrencesFinderJobCanceler;
	/** 
	 * This editor's projection support 
	 * @since 3.0
	 */
	private ProjectionSupport fProjectionSupport;
	/** 
	 * This editor's projection model updater 
	 * @since 3.0
	 */
	private IJavaFoldingStructureProvider fProjectionModelUpdater;
	/**
	 * The override and implements indicator manager for this editor.
	 * @since 3.0
	 */
	protected OverrideIndicatorManager fOverrideIndicatorManager;
	/**
	 * Semantic highlighting manager
	 * @since 3.0
	 */
	private SemanticHighlightingManager fSemanticManager;