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
fCanceled= true;
			cancel();
		}
		
		private boolean isCanceled() {
			return fCanceled || fProgressMonitor.isCanceled()
				||  fPostSelectionValidator != null && !(fPostSelectionValidator.isValid(fSelection) || fForcedMarkOccurrencesSelection == fSelection)
				|| LinkedModeModel.hasInstalledModel(fDocument);
		}
		
		/*
		 * @see Job#run(org.eclipse.core.runtime.IProgressMonitor)
		 */
		public IStatus run(IProgressMonitor progressMonitor) {
			
			fProgressMonitor= progressMonitor;
			
			if (isCanceled())
				return Status.CANCEL_STATUS;
			
			ITextViewer textViewer= getViewer(); 
			if (textViewer == null)
				return Status.CANCEL_STATUS;
			
			IDocument document= textViewer.getDocument();
			if (document == null)
				return Status.CANCEL_STATUS;
			
			IDocumentProvider documentProvider= getDocumentProvider();
			if (documentProvider == null)