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
/**
		 * A property change event filter.
		 */
		public interface IPropertyChangeEventFilter {
			/**
			 * Should the given event be filtered?
			 * @param event The property change event.
			 * @return <code>true</code> iff the given event should be filtered.
			 */
			public boolean isFiltered(PropertyChangeEvent event);
		}
		/**
		 * Property change listener. Listens for events in the options Map and
		 * fires a {@link org.eclipse.jface.util.PropertyChangeEvent}
		 * on this adapter with arguments from the received event.
		 */
		private class PropertyChangeListener implements IPropertyChangeListener {