Null is null or not an object

On every file in wp-admin, when I push a button or try to type in an option I get an error that says.
A Runtime error has occured on Line 50. How can I fix this? When I say to debug it shows this code from edit.php

aInputs[i].onclick = function() {
var inputColl = GetElementsWithClassName('input','valinp');
var rel = document.getElementById('rel');
var inputs = '';
for (i = 0; i < inputColl.length; i++) {
if (inputColl[i].checked) {
if (inputColl[i].value != '') inputs += inputColl[i].value + ' ';
}
}
inputs = inputs.substr(0,inputs.length - 1);
rel.value = inputs;
}

the error happens at rel.value-inputs; This makes using WordPress horrible! Please help.