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/ASTProvider.java
private IJavaElement fReconcilingJavaElement;
	private IJavaElement fActiveJavaElement;
	private CompilationUnit fAST;
	private ActivationListener fActivationListener;
	private Object fReconcileLock= new Object();
	private Object fWaitLock= new Object();
	private boolean fIsReconciling;
	private IWorkbench fWorkbench;
	private IWorkbenchPart fActiveEditor;
	
	/**
	 * Creates a new AST provider. 
	 */
	public ASTProvider() {
		install();
	}
	
	/**
	 * Installs this AST provider.
	 */
	void install() {
		
		fActivationListener= new ActivationListener();
		
		/*
		 * XXX: Don't in-line this field unless the following bug has been fixed:
		 *      https://bugs.eclipse.org/bugs/show_bug.cgi?id=55246
		 */
		fWorkbench= PlatformUI.getWorkbench();