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/OverrideIndicatorManager.java
static final String ANNOTATION_TYPE= "org.eclipse.jdt.ui.overrideIndicator"; 
	private IAnnotationModel fAnnotationModel;
	private Object fAnnotationModelLockObject;
	private Annotation[] fOverrideAnnotations;
	private IJavaElement fJavaElement;
	
	public OverrideIndicatorManager(IAnnotationModel annotationModel, IJavaElement javaElement, CompilationUnit ast) {
		Assert.isNotNull(annotationModel);
		Assert.isNotNull(javaElement);
		
		fJavaElement= javaElement;
		fAnnotationModel=annotationModel; 
		fAnnotationModelLockObject= getLockObject(fAnnotationModel);
		updateAnnotations(ast, new NullProgressMonitor());
	}
	/**
	 * Returns the lock object for the given annotation model.
	 * 
	 * @param annotationModel the annotation model
	 * @return the annotation model's lock object
	 * @since 3.0
	 */
	private Object getLockObject(IAnnotationModel annotationModel) { 
		if (annotationModel instanceof ISynchronizable)
			return ((ISynchronizable)annotationModel).getLockObject();
		else
			return annotationModel;
	}
	/**