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
lineInfo= document.getLineInformationOfOffset(offset);
				line= document.get(lineInfo.getOffset(), lineInfo.getLength());
			} catch (BadLocationException ex) {
				return null;
			}
			
			int offsetInLine= offset - lineInfo.getOffset();
			
			int urlSeparatorOffset= line.indexOf(":"); 
			if (urlSeparatorOffset < 0)
				return null;
			
			
			int urlOffsetInLine= urlSeparatorOffset;
			char ch;
			do {
				urlOffsetInLine--;
				ch= ' ';
				if (urlOffsetInLine > -1)
					ch= line.charAt(urlOffsetInLine);
			} while (!Character.isWhitespace(ch));
			urlOffsetInLine++;
			
			
			StringTokenizer tokenizer= new StringTokenizer(line.substring(urlSeparatorOffset + 3));
			if (!tokenizer.hasMoreTokens())
				return null;
			
			int urlLength= tokenizer.nextToken().length() + 3 + urlSeparatorOffset - urlOffsetInLine;
			if (offsetInLine < urlOffsetInLine || offsetInLine > urlOffsetInLine + urlLength)