Adding Physics Joints Throws an Error

Hello, I am currently designing a pinball game and I am trying to implement the flippers that hit the pinball. When I tried to add a physics joint to each flipper to make it rotate or “flip” it gives me an error because my physics world is null. After researching this I discovered that you can not add physics joints in layer. Unfortunately I could not find any forums that were clear in how to handle this. If my game is only one scene added to layer is their a method I can use to add my joints after the physics world has started?