That's a nice thought and all, but each application controls the mouse cursor when the mouse is over their windows. You can try to change it, but don't expect that change to stick or work in every situation.
You can change the cursor at the system level, basically, the "default" cursor shape, but don't expect it to stay that way when the mouse moves over an application window.
Marco has clearly identified the most probable source of your 'null error.
Any time you have an error like this, it's a good idea to set a break-point before the error occurs, and then single-step through the code, inspecting the value of variables, the values returned by methods, etc.
"What Turing gave us for the first time (and without Turing you just couldn't do any of this) is he gave us a way of thinking about and taking seriously and thinking in a disciplined way about phenomena that have, as I like to say, trillions of moving parts.
Until the late 20th century, nobody knew how to take seriously a machine with a trillion moving parts. It's just mind-boggling." Daniel C. Dennett