Why does this function still reference a destroyed child?

Questions & Answers
1

[Initial question deleted, it was not the root of the problem, see edit.]

Edit: Quick explaination, this script is one that assigns a cursor object to the mouse. On selection of a new cursor object this script is called, where the old cursorobject is deleted and a new one added: it would seem that the problem actually lies with the previous “mousecursor” object, which is getting destroyed, is still being referenced?:

public void SetObjToPlace(GameObject obj)
                 {
                     objToPlace = obj;
                     cursorChildren = GetComponentsInChildren<Transform>();
             
                     // Clear collisionScripts
                     collisionScripts.Clear();
             
                     // Delete old object cursor
                     if (cursorChildren != null)
                     {
                         foreach (Transform child in transform)
                         {
                             Destroy(child.gameObject);
   //  ^ THIS OBJECT STILL GETS REFERENCED IN AddRigidBodyAndTriggerToAllChildren
                         }
                         cursorChildren = GetComponentsInChildren<Transform>();
                     }
             
                     // Create new object cursor
                     GameObject newCursor = Instantiate(obj);
                     newCursor.transform.parent = transform;
             
                     AddRigidBodyAndTriggerToAllChildren(transform);
            // ^ This ^ function throws the error "Can't add component 'Rigidbody' to NameOfPreviouslyDestroyedObject because such a component is already added to the game object (well no shit, but it shouldn't check that destroyed child)
             
                     transform.rotation = Quaternion.identity;
                     newCursor.transform.position = transform.position;
                     SetChildShader();
                 }
        
            private void AddRigidBodyAndTriggerToAllChildren(Transform parent)
                {
                    foreach (Transform child in parent)
                    {
                        BoxCollider[] childColliders = child.gameObject.GetComponents<BoxCollider>();
                        if (childColliders.Length > 0)
                        {
                            Rigidbody childRB = child.gameObject.AddComponent<Rigidbody>();
        // ^ This is the line throwing the error ^
                            childRB.useGravity = false;
        // ^ Causing a nullref for childRB here 
                            childRB.isKinematic = true;
            
                            PlacementCollision collisionScript = child.gameObject.AddComponent<PlacementCollision>();
                            collisionScripts.Add(collisionScript);
            
                            foreach (BoxCollider collider in childColliders)
                            {
                                collider.isTrigger = true;
                            }
                        }
            
                        AddRigidBodyAndTriggerToAllChildren(child);
                    }
                }
2

A simpler way to do what you’re trying, while also ensuring transforms aren’t referenced more than once, is to use GetComponentsInChildren.

            // This retrieves children, grandchildren, great-grandchildren, etc.
            Transform[] children = GetComponentsInChildren<Transform>();
            foreach (Transform child in children)
            {
                // Execute code with child
            }

Otherwise you can store the child Transforms already collected in a List, then check if the child has already been added to the list before performing a function.