I propose 2 new functions.

DictionaryAdd

If the dictionary item exists, it is overwritten.

  <function name="DictionaryAdd" parameters="dict, key, val">
    if (dict = null) {
      Log ("DictionaryAdd ERROR:  Dictionary does not exist!")
      return (false)
    }
    if (not DictionaryContains(dict,key)) {
      dictionary add (dict, key, val)
    }
    else {
      if (not DictionaryItem(dict,key) = val) {
        dictionary remove (dict, key)
        dictionary add (dict, key, val)
      }
    }
  </function>

DictionaryRemove

Checks for the dictionary item's existence before removing it.

  <function name="DictionaryRemove" parameters="dict, key">
    if (dict = null) {
      Log ("DictionaryAdd ERROR:  Dictionary does not exist!")
      return (false)
    }
    if (DictionaryContains(dict,key)) {
      dictionary remove (dict, key)
    }
  </function>

In both scripts, if the dictionary parameter is null, an entry is added to the log (rather than throwing an error *), and the script exits.