Your hierarchy is correct. That problem surely comes from some script trying to configure functions at the WheelColliders prior to the rigidbody to be initialized.
Try setting up an empty scene containing a car with same hierarchy and no scripts at all. There should be no error here. Add your scripts one by one until you receive the error. Then try commenting out portions of the offending script until you find the exact code that causes the problem.
Following should solve ur problem
suppose goCar is gameobject referring to Car (root of ur hierarchy), then while activating/re-activating the gameobject do this:
goCar.active = true;
goCar.SetActiveRecursively(true); // now wheel colliders will be able to find rigidbody since it has been made available in previous line
First line ensures that the rigidbody is already activated for the children activated by the second line.