Trail prefab as the child of existing object.

Hello, developers. I tried to make a trail of an explosion, which would apper in the point of collision and would be child to the object which was hit. My code works if and only if I implement it with primitive object. Once I try to put a prefab or a model manually, then I get either "object reference not set to an instance of an object" or "Setting the parent of a transform which resides in a prefab is disabled to prevent data corruption".

Here are two codes.

1. With primitives, works:

           void OnCollisionEnter (Collision collision){
        if (collision.gameObject.name == "Sphere(Clone)") {        
            ContactPoint contactPoint = collision.contacts[0];        
            GameObject cube =  GameObject.CreatePrimitive(PrimitiveType.Cube);
            cube.transform.position = contactPoint.point;
            cube.transform.rotation = Quaternion.FromToRotation(Vector3.up, contactPoint.normal);
            cube.transform.parent = transform;
        }
    }
   
   
   

2. With prefab (g1), gives "object reference not set...":

             void OnCollisionEnter (Collision collision){
              if (collision.gameObject.name == "Sphere(Clone)") {        
            ContactPoint contactPoint = collision.contacts[0];        
            
            GameObject cube = Instantiate(g1, contactPoint.point,    Quaternion.FromToRotation(Vector3.up, contactPoint.normal)) as GameObject;
            
            cube.transform.parent = transform;
        }
    }