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
parser.setResolveBindings(true);
		
		if (je.getElementType() == IJavaElement.COMPILATION_UNIT)
			parser.setSource((ICompilationUnit)je);
		else if (je.getElementType() == IJavaElement.CLASS_FILE)
			parser.setSource((IClassFile)je);
		
		if (progressMonitor != null && progressMonitor.isCanceled())
			return null;
		try {
			CompilationUnit root= (CompilationUnit)parser.createAST(progressMonitor);
			
			ASTNodes.setFlagsToAST(root, ASTNode.PROTECT);
			return root;
		} catch (IllegalStateException ex) {
			return null;
		}
	}
		
	/**
	 * Disposes this AST provider.
	 */
	public void dispose() {
		
		fWorkbench.removeWindowListener(fActivationListener);
		fActivationListener= null;
		
		disposeAST();
		
		synchronized (fWaitLock) {