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
* @since 3.0
	 */
	private Annotation getAnnotation(int offset, int length) {
		IAnnotationModel model= getDocumentProvider().getAnnotationModel(getEditorInput());
		Iterator e= new JavaAnnotationIterator(model, true, true);
		while (e.hasNext()) {
			Annotation a= (Annotation) e.next();
			if (!isNavigationTarget(a))
				continue;
				
			Position p= model.getPosition(a);
			if (p != null && p.overlapsWith(offset, length))
				return a;
		}
		
		return null;
	}
	
	/**
	 * Returns whether the given annotation is configured as a target for the
	 * "Go to Next/Previous Annotation" actions
	 * 
	 * @param annotation the annotation
	 * @return <code>true</code> if this is a target, <code>false</code>
	 *         otherwise
	 * @since 3.0
	 */
	private boolean isNavigationTarget(Annotation annotation) {
		Preferences preferences= EditorsUI.getPluginPreferences();
		AnnotationPreference preference= getAnnotationPreferenceLookup().getAnnotationPreference(annotation);