Did you try Kimimaru's suggestion? If I'm reading this right, it's saying that 'Game.Content.get' is null, which means that the
Content : ContentManager property in the Game class was never set. That sort of thing should normally happen during initialization of the game. If the Game class hasn't finished initializing when this method is called, this error might be thrown as it hasn't had a chance to set that property yet.
You could test this theory by just making a new ContentManager instance before trying to use 'Content'.
this.Content = new ContentManager(Services, "yourDirectoryHere");
This may be an okay workaround.