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
/**
	 * Returns the boolean preference for the given key.
	 * 
	 * @param store the preference store
	 * @param key the preference key
	 * @return <code>true</code> if the key exists in the store and its value is <code>true</code>
	 * @since 3.0
	 */
	private boolean getBoolean(IPreferenceStore store, String key) {
		return key != null && store.getBoolean(key);
	}
	
	/**
	 * Determines whether the preference change encoded by the given event
	 * changes the override indication.
	 * 
	 * @param event the event to be investigated
	 * @return <code>true</code> if event causes a change
	 * @since 3.0
	 */
	protected boolean affectsOverrideIndicatorAnnotations(PropertyChangeEvent event) {
		String key= event.getProperty();
		AnnotationPreference preference= getAnnotationPreferenceLookup().getAnnotationPreference(OverrideIndicatorManager.ANNOTATION_TYPE);
		if (key == null || preference == null)
			return false;
		
		return key.equals(preference.getHighlightPreferenceKey())
			|| key.equals(preference.getVerticalRulerPreferenceKey())
			|| key.equals(preference.getOverviewRulerPreferenceKey())