After Instantiating a Prefab I get a NullReference Error even though the Object was created

So I don´t know if I´m missing an dumb error of myself but my problem is: I´ve got an class which deserializes a json file into a tree-structure. That part is working without an error. Now while doing this i want to create Objects of another class - we just call it "CityObject" for the moment.

 public class CityObject : MonoBehaviour {
  
     private GameObject gameObject;
  
     public ArrayList children;
  
     public int width;
  
  
     public CityObject(GameObject prefab, int newWidth)
     {
         gameObject = Instantiate(prefab);
         children = new ArrayList();
         width = newWidth;
     }
 }

Depending on the Information in the json file, the GameObject should get a simple cube prefab or a flat cube prefab and instantiate it. So far so good - in the unity Scene all Objects are created, so in my example there are 5 created GameObjects - 2 flat and 3 simple cubes. Code (CSharp):

 public class CityBuilder : MonoBehaviour {
  
     public GameObject plane;
     public GameObject cube;
     public CityObject origin;
  
     private int maxW = 0;
  
     private ProjectView ProcessProjectData(string jsonString)
     {
         ProjectView parsejson = JsonUtility.FromJson<ProjectView>(jsonString);
         return parsejson;
     }
  
     private GameObject FindPrefab(string type)
     {
         switch (type.ToLower())
         {
             case "block": newPlane = false; return cube;
             case "scope": newPlane = true; return plane;
             default: return null;
         }
     }
    
     private CityObject CreateGameObjectFromProjectData(ProjectData projectData, CityObject cityObjectParent)
     {
      
         var prefab = FindPrefab(projectData.type);
         if (!prefab)
             return null;
  
  
         CityObject rootCityObject = new CityObject(prefab,  getSubtreeWidth(projectData));
  
        
         if(maxW == 0)
         {
             maxW = rootCityObject.width;
         }
         scaleObject(rootCityObject);
        
         /* * *
          * If an object has one or more children, then the children should execute this method on their own.
          */
  
         if (projectData.children != null)
         {
             foreach (ProjectData data in projectData.children)
             {
                 Debug.Log(data.id);
             }
             foreach (ProjectData data in projectData.children)
             {
                 CityObject newChild = CreateGameObjectFromProjectData(data, rootCityObject);
                 rootCityObject.appendChild(newChild);
             }
         }
         return rootCityObject;
     }

The problem is that in line 40 i want to scale the gameObject of the cityObject like this:

 private void scaleObject(CityObject node)
 
     {
 
         float scaleRatio = (float)(node.width/maxW);
 
         node.gameObject.transform.localScale.Set(scaleRatio, scaleRatio, scaleRatio);
 
     }

Then there is a NullReference Error for the node.GameObject - so there seems to be no reference between the cityObject and the GameObject. This is the part i can´t understand. Is it the Instantiate in the CityObejct Constructor? Isn´t the Instantiate creating a Reference between the "Script-Object" and the Gameobject in the unity-Scene?

Just for explanation. I don´t want to create a GameObject in the scene which should get the "CityObject" Component. What i wanna do is creating an CityObject (just in code) and this CityObject has a "gameObject" attribute which is basically just a real GameObject in the scene. After creating it, i still want to change the GameObject. I want to scale it and later on i want to reposition it but when I try to access it, there´s the NullReference Error.