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/viewsupport/SelectionListenerWithASTManager.java
private ITextEditor fPart;
		private ISelectionChangedListener fSelectionListener, fPostSelectionListener;
		private Job fCurrentJob;
		private ListenerList fAstListeners;
		/**
		 * Lock to avoid having more than one calculateAndInform job in parallel.
		 * Only jobs may synchronize on this as otherwise deadlocks are possible.
		 */
		private final Object fJobLock= new Object();
		
		public PartListenerGroup(ITextEditor part) {
			fPart= part;
			fCurrentJob= null;
			fAstListeners= new ListenerList();
			
			fSelectionListener= new ISelectionChangedListener() {
				public void selectionChanged(SelectionChangedEvent event) {
					ISelection selection= event.getSelection();
					if (selection instanceof ITextSelection) {
						fireSelectionChanged((ITextSelection) selection);
					}
				}
			};
			
			fPostSelectionListener= new ISelectionChangedListener() {
				public void selectionChanged(SelectionChangedEvent event) {
					ISelection selection= event.getSelection();
					if (selection instanceof ITextSelection) {