Because the error says "null or not an object".
Code:
<!-- //
conv_tbl = [
  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,
  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,
  32,  33,8704, 1513,1505,  37,  83,8717,  65, 79,  8727, 1496,  44,8722,  58,  47,
  48,  49,  50,  51,  52,  53,  54,  55,  56,  57,  46,  59,  60,  61,  62,  63,
  64, 1488, 1489, 1510, 1491, 69, 70, 1490, 1492, 73, 74, 1499, 1500, 1502, 1504, 1506,
  1508, 81, 1512, 1513, 1514, 85, 1493, 87, 1495, 1497, 90, 91, 92,  93, 94,  95,
  63, 97, 66, 1509, 68, 101, 102, 71, 72, 105, 106, 1498, 1500, 1501, 1503, 111,
  1507, 1511, 82, 8707, 84, 117, 118, 86, 88, 89, 1494, 123, 124, 125, 126,  63,
  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,
  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,
  63, 978,8242,8804,8260,8734, 402,9827,9830,9829,9824,8596,8592,8593,8594,8595,
  63, 177,8243,8805, 215,8733,8706,8729, 247,8800,8801,8776,8230,  63,  63,8629,
  8501,8465,8476,8472,8855,8853,8709,8745,8746,8835,8839,8836,8834,8838,8712,8713,
  8736,8711, 174, 169,8482,8719,8730,8901, 172,8743,8744,8660,8656,8657,8658,8659,
  9674,9001, 174, 169,8482,8721,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63,
  63,9002,8747,8992,  63,8993,  63,  63,  63,  63,  63,  63,  63,  63,  63,  63
];

function convert(mc, unic_textarea)
{
   unic = '';

   for (i = 0; i < mc.length; i++)
   {
      tbl_idx = mc.charCodeAt(i);
      code    = tbl_idx < conv_tbl.length ? conv_tbl[tbl_idx] : 63;
      unic    = unic + String.fromCharCode(code)
   }

   unic_textarea.value = unic;
}

function mc2unic()
{
   mc = document.conv_form.mc_code.value;
   convert(mc, document.conv_form.unic_area);
}

// -->