Getting the unique identifier from a given login username
catch(Exception)
{
    return null;
}


You should never do this. Exception means that there is some sort of problem. And you shouldn't ignore problems, you should learn about them and do something about them as soon as possible.

What you should do is that at first, don't catch any exceptions. Then, if you find out that some exceptions actually do happen under normal conditions, then handle those, but only those.