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/viewsupport/SelectionListenerWithASTManager.java
* @param listener The listener to unregister.
	 */
	public void removeListener(ITextEditor part, ISelectionListenerWithAST listener) {
		synchronized (this) {
			PartListenerGroup partListener= (PartListenerGroup) fListenerGroups.get(part);
			if (partListener != null) {
				partListener.uninstall(listener);
				if (partListener.isEmpty()) {
					fListenerGroups.remove(part);
				}
			}
		}
	}
	
	/**
	 * Forces a selection changed event that is sent to all listeners registered to the given editor
	 * part. The event is sent from a background thread: this method call can return before the listeners
	 * are informed.
	 * @param part The editor part that has a changed selection
	 * @param selection The new text selection
	 */
	public void forceSelectionChange(ITextEditor part, ITextSelection selection) {
		synchronized (this) {
			PartListenerGroup partListener= (PartListenerGroup) fListenerGroups.get(part);
			if (partListener != null) {
				partListener.firePostSelectionChanged(selection);
			}
		}
	}}