Error in illustrator regex text replace script

I tend to over-complicate things so you may want a simpler solution, but if I were to rewrite the function you've shown me I would do it like this:

var options = {
  // Bypass all find/replace prompts, just remove the newlines:
  removeNewlines: false,
  //
  find: {
    value: "[\\r\\n\\cC]", // Text to appear as default
    title: "Find:", // Title of window
    desc: "Enter RegExp/GREP/Text", // Description of window
    flags: "gm",
  },
  replace: {
    value: "",
    title: "Replace:",
    desc: "Enter Text",
  },
  confirm: {
    value: false,
    title: "Enable line-splitting?",
    desc: "Choose no to enable global flags and newline targeting",
  },
  lineSplitting: {
    onSplit: /[\r\n\cC]/gm, // RegExp to use for manual newline split
    onJoin: "\r\n", // Value to join newlines back with
  },
  showChanges: true, // Whether to display alert on completion noting change count
};

Array.prototype.filter = function(callback) {
  var filtered = [];
  for (var i = 0; i < this.length; i++)
    if (callback(this[i], i, this)) filtered.push(this[i]);
  return filtered;
};

function get(type, parent) {
  if (arguments.length == 1 || !parent) parent = app.activeDocument;
  var result = [];
  if (!parent[type]) return result;
  for (var i = 0; i < parent[type].length; i++) result.push(parent[type][i]);
  return result;
}

function findReplace() {
  try {
    var doc = app.activeDocument,
      scope = doc.selection.length
        ? get("selection").filter(function(i) {
            return i.typename == "TextFrame";
          })
        : doc.textFrames,
      count = 0;
    if (options.removeNewlines) {
      for (var i = 0; i < scope.length; i++) {
        var content = scope[i].contents;
        var lines = content.split(/[\r\n\cC]/gm),
          newStr = [];
        for (var e = 0; e < lines.length; e++)
          newStr.push(lines[e].replace(find, replace));
        scope[i].contents = newStr.join(" ").replace(/\s{2,}/gm, "");
      }
      return null;
    }
    var find = prompt(
      options.find.desc,
      options.find.value,
      options.find.title
    );
    if (find !== null) {
      var findRX = new RegExp(find, options.find.flags),
        shouldSplit = confirm(
          options.confirm.desc,
          !options.confirm.value,
          options.confirm.title
        );
      var replace = prompt(
        options.replace.desc,
        options.replace.value,
        options.replace.title
      );
      if (replace !== null) {
        for (var i = 0; i < scope.length; i++) {
          var content = scope[i].contents;
          if (shouldSplit) {
            var lines = content.split(options.lineSplitting.onSplit),
              newStr = [];
            for (var e = 0; e < lines.length; e++) {
              if (findRX.test(lines[e])) count++;
              newStr.push(lines[e].replace(findRX, replace));
            }
            scope[i].contents = newStr.join(options.lineSplitting.onJoin);
          } else {
            if (findRX.test(scope[i].contents)) count++;
            scope[i].contents = scope[i].contents.replace(findRX, replace);
          }
        }
        if (options.showChanges) {
          var alertStr = !count
            ? "No changes made"
            : count > 1
            ? count + " changes made"
            : "1 change made";
          alert(alertStr);
        }
      }
    }
  } catch (err) {
    alert("Error: " + err);
  }
}
findReplace();

With options.removeNewLines set, there is no UI. It just takes this:

And turns it into this:

Otherwise all the option values at the top of the script determine the behavior and UI:

The above defaults to No, which targets the "paragraph" and all lines for a single RegExp replace. When choosing Yes, it will split each line into a sentence and run an individual replace on each sentence, then reconstruct and assign the value back to the original TextFrame. All this behavior is configurable through the options object at the top of the script.