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 String toString(CompilationUnit ast) {
		if (ast == null)
			return "null"; 
		
		List types= ast.types();
		if (types != null && types.size() > 0)
			return ((AbstractTypeDeclaration)types.get(0)).getName().getIdentifier();
		else
			return "AST without any type"; 
	}
	/**
	 * Caches the given compilation unit AST for the given Java element.
	 * 
	 * @param ast
	 * @param javaElement
	 */
	private synchronized void cache(CompilationUnit ast, IJavaElement javaElement) {
		
		if (DEBUG && (javaElement != null || ast != null)) 
			System.out.println(DEBUG_PREFIX + "caching AST:" + toString(ast) + " for: " + toString(javaElement)); 
		if (fAST != null)
			disposeAST();
		fAST= ast;
		
		fActiveJavaElement= javaElement;
		
		
		synchronized (fWaitLock) {
			fWaitLock.notifyAll();
		}