How to create a 2D Polygon Sprite from prefab at runtime?

Question

At start of my 2D unity game I want to create a new "building" (at random).

So I created a prefab named "Building" from a polygon sprite. Then I created a "Empty Game Object" named "GameManager" with a script named "GameManager" attached to it. The script looks like this:

public class GameManager : MonoBehaviour
{
    public GameObject Building;
    // Start is called before the first frame update
    void Start()
    {
        var createdBuilding = Instantiate(Building, new Vector3(0, 0, 0), Quaternion.identity);
        var sprite = createdBuilding.GetComponent<SpriteRenderer>().sprite;
        Vector2[] geometry = new Vector2[6];
        geometry[0] = new Vector2(0,0);
        geometry[1] = new Vector2(0,20);
        geometry[2] = new Vector2(20,20);
        geometry[3] = new Vector2(27,20);
        geometry[4] = new Vector2(29,29);
        geometry[5] = new Vector2(16,0);
        sprite.OverrideGeometry(geometry, sprite.triangles);
    }
}

The hardcoded position values are just for testing purporses. The plan is to create multible buildings but first I have to get unity to show at least one of them.

Afterwards I attached the prefab "Building" to the variable "Building" via the inspector.

But when I ran the game the sprite is shown as a Pentagon, regardless what I write into the "geometry"-Array.

So why I cant change the geometry? And is this even the right way to create buildings at startup?