First, find out which bit is null.
Replace that line with this:
var x1 = xmlDocument.Element("result");
var x2 = x1.Element("iban")'
iban = x2.ToString();
And run your code in the debugger.
Put a breakpoint on the first of those three lines, and when it hits, check the
value. If it's null, you can look for why.
If it isn't, single step the line, and check the result. If that's null, ... you get the idea.
Find out where the null reference is and what is causing it, then you can look at why it's happening - then you can start to fix it, but not until then!