Hi XanderTodor-5936,
The type of error System.NullReferenceException 'Object reference not set to an instance of an object.' in functional testing is usually thrown when the element defined in the code does not actually exist on the page, so the action cannot be sent to it. A good coding practice is to use an Assert statement after defining an element to check if that new variable is not null in test run-time.
If you use Test Studio to record the UI web functional tests, you can rely on the DOM Explorer tab in the Advanced Recording Tools - there you can see the complete HTML output of the loaded page and find the attributes of each element. On top of this Test Studio automatically generates find expression for each added element by using the unique attributes of the HTML elements.
I hope this can help you in setting up your automation tests.
Cheers!