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
return null;
			IJavaElement input= SelectionConverter.getInput(JavaEditor.this);
			if (input == null)
				return null;
			try {
				
				IJavaElement[] elements= null;
				synchronized (input) {
					elements= ((ICodeAssist) input).codeSelect(offset, 0);
				}
				
				IDocument document= viewer.getDocument();
				if (elements == null || elements.length == 0)
					return findAndSetURLText(document, offset);
				else
					return selectWord(document, offset);
					
			} catch (JavaModelException e) {
				return null;	
			}
		}
		
		private IRegion findAndSetURLText(IDocument document, int offset) {
			fURLString= null;
			if (document == null)
				return null;
			
			IRegion lineInfo;
			String line;
			try {