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
private IPropertyChangeEventFilter fFilter;
		
		/**
		 * Initialize with the given options.
		 * 
		 * @param options The options to wrap
		 * @param mockupPreferenceStore the mock-up preference store
		 * @param filter the property change filter
		 */
		public OptionsAdapter(Map options, IPreferenceStore mockupPreferenceStore, IPropertyChangeEventFilter filter) {
			fMockupPreferenceStore= mockupPreferenceStore;
			fOptions= options;
			setFilter(filter);
		}
		/**
		 * {@inheritDoc}
		 */
		public void addPropertyChangeListener(IPropertyChangeListener listener) {
			if (fListeners.size() == 0)
				fMockupPreferenceStore.addPropertyChangeListener(fListener);
			fListeners.add(listener);
		}
		/**
		 * {@inheritDoc}
		 */
		public void removePropertyChangeListener(IPropertyChangeListener listener) {
			fListeners.remove(listener);
			if (fListeners.size() == 0)
				fMockupPreferenceStore.removePropertyChangeListener(fListener);
		}