I have the same issue. If I just create a fresh monogame (MonoMac) solution, I don’t have any issues yet. Despite installing 3.4, the version listed under the Packages folder says 3.3.0 and indicates that there’s an update available. When I tell it to go ahead and update to 3.4, that’s when I run into the same issue as you. Have you found a solution?
It doesn’t. What it does do though is instead of the expected blue screen it’s a hung grey one. Even clicking the stop button in Xamarin Studio doesn’t stop it from running; I have to force quit. However, if I get rid of Gamepad and Joystick, leaving in just the Keyboard check – no null exception.
When I created a new solution, the problem went away, but I noticed the solution was using MonoGame 3.3.0. As soon as I updated to 3.4, the crash returns.
I also got a chance to try Joystick.GetState(). I used the default project and just swapped out the gamepad call for a joystick one. I am seeing the same behavior as sammyw. I get a grey screen that hangs that I have to force quit. No NRE.