NullReferenceException and don't know how to fix it - unity3d

Question

Okay, I am making a simple game mechanic where you are a ball rolling along a small panel. On the edge of the panel are 8 child objects. 4 of them are triggers on the edges of the panel, and 4 of them are empty game objects 1 unit away from each edge of the panel for the location of the next panel prefab to spawn at. The ball has a trigger on it that detects the location of the empty game objects to tell the panel prefab where to spawn. When the ball enters a specific trigger frm the panel the ball is suppose to instantiate a panel prefab on the location that I assign based on the trigger the ball enters. Here is my code:
public GameObject panelPrefab;
Transform frontSpawn;
Transform backSpawn;
Transform leftSpawn;
Transform rightSpawn;
private bool allowSpawn;
void Awake()
{
allowSpawn = true;
}
void OnTriggerStay(Collider spawn)
{
if (spawn.gameObject.tag == "FrontSpawn")
{
frontSpawn = spawn.transform;
}
else if (spawn.gameObject.tag == "BackSpawn")
{
backSpawn = spawn.transform;
}
else if (spawn.gameObject.tag == "LeftSpawn")
{
leftSpawn = spawn.transform;
}
else if (spawn.gameObject.tag == "RightSpawn")
{
rightSpawn = spawn.transform;
}
}
void OnTriggerEnter (Collider other)
{
if (other.gameObject.tag == "Front" && allowSpawn == true)
{
Instantiate (panelPrefab, frontSpawn.transform.position, Quaternion.identity);
allowSpawn = false;
}
else if (other.gameObject.tag == "Back" && allowSpawn == true)
{
Instantiate (panelPrefab, backSpawn.transform.position, Quaternion.identity);
allowSpawn = false;
}
else if (other.gameObject.tag == "Left" && allowSpawn == true)
{
Instantiate (panelPrefab, leftSpawn.transform.position, Quaternion.identity);
allowSpawn = false;
}
else if (other.gameObject.tag == "Right" && allowSpawn == true)
{
Instantiate (panelPrefab, rightSpawn.transform.position, Quaternion.identity);
allowSpawn = false;
}
}
void OnTriggerExit (Collider other)
{
allowSpawn = true;
}
My issue is on each of the Instantiate calls, I am getting a NullReferenceException. I have the panelPrefab assigned in the unity editor, and I don't know what could be causing this! If anyone can help me here it would be GREATLY appreciated... So thank you in advance!

Answer 1

OnTriggerEnter is called before OnTriggerStay. The error is not due to the panelPrefab object. It might happen that your rightSpawn, leftSpawn etc. objects are null and hence cannot access the transform property of a null object.
Before instantiating verify if rightSpawn etc. is null or not and then access it's position.