Spawning GameObject contained within another

[system]

Here is what I am trying to do. I have a gameobject. When a box collides with it, it should spawn another gameobject. I followed this link http://unity3d.com/support/documentation/Manual/Instantiating%20Prefabs.html . My code is pasted below:

var purpleTriangleBottomOuterCornerMelty : GameObject;
function OnCollisionEnter(event : Collision) {
    Debug.Log("collision " + name);
    createMelty(name, transform.position);
}

function createMelty(objectName: String, objectCoords: Vector3) {
    Debug.Log("instantiating " +  meltyMappingList[objectName]["rotation"]);
    Instantiate(purpleTriangleBottomOuterCornerMelty, objectCoords, Quaternion.Euler(0, meltyMappingList[objectName].rotation, 0));
}

I have set the purpleTriangleBottomOuterCornerMelty variable to the prefab that I want to spawn, in the inspector of the object containing this script. But the Instantiate gives me a NullReferenceException. Apologies if I've missed something obvious, but I'm a total noob.

PS The above script is in my spawning object. I have a number of spawing objects, and want to spawn a different GameObject depending on the spawning object, so I made a mapping object (not included in the code) to map spawning object name to spawnee object.

[loopyllama]

To me, it looks like this:

meltyMappingList[objectName]["rotation"])

and this:

meltyMappingList[objectName].rotation 

are what I would concentrate on. Something isn't quite right with those two. You didn't include what the meltyMappingList is, but how can meltyMappingList[string][string] and meltyMappingList[string].rotation both be correct? Are you sure you didn't want something like:

meltyMappingList[int]+ ".rotation")

and:

meltyMappingList[int].rotation