I'm not sure why you're setting the BodyObject in this line?
wallLeft.BodyObject = new FarseerPhysics.Dynamics.Body(cnvGame.Simulator);
The bodyobject is set by the physics helper for you when the control is loaded into the canvas so you shouldn't need this.
Also, what is relativeHeight? (and width is pretty skinny, unless you are using Bullets everywhere things could pass through it).
But that's just a couple of things I noticed, not sure what is going on. If you still have problems, can you post the line of code that is raising the null reference exception, and the exception details?