Cannot read property of 'firstChild' error

[championcyclones]

I keep getting this error message: Uncaught TypeError: Cannot read property 'firstChild' of undefined filterScene

I am trying to hide portions of the scene when the select list is selected.

function addEvent(object, evName, fnName, cap) {
   if (object.attachEvent)
       object.attachEvent("on" + evName, fnName);
   else if (object.addEventListener)
       object.addEventListener(evName, fnName, cap);
}


function uniqueElemText(elemName) {
   elems = document.getElementsByTagName(elemName);
   elemsArray = new Array();

   for (var i=0; i<elems.length; i++) elemsArray[i]=elems[i].innerHTML;  
   elemsArray.sort();
   for (i=0; i<elemsArray.length-1; i++) {
      if (elemsArray[i]==elemsArray[i+1]) {
         elemsArray.splice(i+1,1);
         i--;
      }
   }
   return elemsArray;
}

addEvent(window, "load", addCharList, false);

var sourceDoc; // document on which the character list is based

function addCharList() {
 newP = document.createElement("p");
 newP.innerHTML = "Show Only Lines By:";
 document.getElementById("characterList").appendChild(newP);

 selectList = document.createElement("select");
 selectList.id="cList";
 selectList.onchange=filterScene;

 newOption = document.createElement("option");
 newOption.innerHTML="Show All Character Lines";
 selectList.appendChild(newOption);

 var characters = uniqueElemText("h3");
 for (i=0; i<characters.length; i++) {
 newOption = document.createElement("option");
 newOption.innerHTML=characters[i];
 selectList.appendChild(newOption);
 }

 document.getElementById("characterList").appendChild(selectList);
}

function filterScene() {
 var displayStatus = "";
 for (var n = sourceDoc.firstChild; n != null; n = n.nextSibling) {
 var optionEntry = document.getElementById("cList");
 if (isHidden(optionEntry)) displayStatus = "none"
 else displayStatus = "";

 if (n.nodeType == 1) { // node represents a page element
 // apply the current display status to the node
 n.style.display = displayStatus;
 }

 }
}

function isHidden(object) {
 for (var n = object; n.nodeName != "BODY"; n = n.parentNode) {
 if (n.style.display == "none") return true;
 }
 
return false;
}

Any help would be greatly appreciated!