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/SemanticHighlightingReconciler.java
fSourceViewer= null;
		fSemanticHighlightings= null;
		fHighlightings= null;
		fPresenter= null;
	}
	/**
	 * Schedule a background job for retrieving the AST and reconciling the Semantic Highlighting model.
	 */
	private void scheduleJob() {
		final IJavaElement element= fEditor.getInputJavaElement();
		if (element != null) {
			Job job= new Job(JavaEditorMessages.getString("SemanticHighlighting.job")) { 
				protected IStatus run(IProgressMonitor monitor) {
					synchronized (fJobLock) {
						if (fJob != null)
							try {
								fJobLock.wait();
							} catch (InterruptedException e) {
								JavaPlugin.log(e);
							}
						if (fCancelJobs || fJob != null)
							return Status.CANCEL_STATUS;
						fJob= this;
					}
					CompilationUnit ast= JavaPlugin.getDefault().getASTProvider().getAST(element, true, monitor);
					reconciled(ast, false, monitor);
					synchronized (fJobLock) {
						fJob= null;
						fJobLock.notifyAll();
					}