XQueryReader bug

In short XQueryReader sometimes return empty string inspite of value which exists in XML string. I attach simple application to show this. This app receives XML file from Google which contain latitude and longitude of typed city (only these two values are extracted from the file). It gives also latitude and longitude for another city. And receives XML with elevation points between these cities. At last it prints chart with elevations. I admit that in my system strange errors occurs so I can't guarante that it is purely Qt bug (maybe it is my system bug). So I attach screenshot with error also. To observe this bug please compile and run this app, then fast clicking on button "Make elevation chart". I hope error will appear in few seconds like on my system (please use mouse instead keyboard to avoid accidental closing error message when it suddenly appear).