For loop and instantiate problems,void Start() running multiple times

I have a script that looks like this:
void grid() { for (int x = 0; x < gridLength; x++) { for (int y = 0; y < gridHeight; y++) { Debug.Log(x + ", " + y); Instantiate(prefab, new Vector3(x - gridLength/2, y - gridHeight/2, 0), Quaternion.identity); } } } private void Start() { grid(); Destroy(gameObject); }
Which does draw a grid but outputs the following several million times:
0, 0 InvalidCastException: Specified cast is not valid. (wrapper castclass) System.Object.__castclass_with_cache(object,intptr,intptr) ,`void grid() { for (int x = 0; x < gridLength; x++) { for (int y = 0; y < gridHeight; y++) { Debug.Log(x + ", " + y); Instantiate(prefab, new Vector3(x - gridLength/2, y - gridHeight/2, 0), Quaternion.identity); } } } private void Start() { grid(); Destroy(gameObject); }` Firstly, this loop runs forever somehow. Secondly, the output window contains
0, 0 InvalidCastException: Specified cast is not valid. (wrapper castclass) System.Object.__castclass_with_cache(object,intptr,intptr) a million times