Custom Action Won't Compile

I have this code I need so I can get a list of JSONs to use in an API call to create multiple rows at once.

I tried looking in Docs but don't see anything relevant (maybe I'm just not finding the right page).

UPDATE: have now tried changing all use of the custom Data Type to JSON instead, and the error is ultimately the same.

Info on the custom functions & data type used:

It calls two functions that FlutterFlow is happy with. But one returns a custom data type (because FlutterFlow would not allow me to return a List<List<String>> as I tried at first, so I just made a "listModel" DataType - which is a string value called "name", and a list<string> value called "list").

The first custom function: takes a string& returns a list of strings.

The second custom function: takes a list<string> and returns a list<ListModelStruct>. ( Just splits each string in the passed list into it's own list of strings). Update: It now returns a JSON with a list inside.

Here is the code:

List<Map<String, dynamic>> singleJsonToMultiJson(
  dynamic inJson,
  String inItemSep,
  String inPartSep,
  String style,
  {bool allowEmptyStrings = true}
) {
  // Extract the original values of "name" and "main" from inJSON
  String ogName = inJson['name'];
  String ogMain = inJson['main'];

  // use splitStringToItemList and pass in ogName/ogMain and inItemSep
  List<String> nameItems = splitStringToItemList(ogName, inItemSep);
  List<String> mainItems = splitStringToItemList(ogMain, inItemSep);

  // List to return later
  List<Map<String,dynamic>> jsonList = [];

  // CHECK Style
  if (style == 'strings') {
  // Return = list of JSONs with strings for Name & Main
    for (int i = 0; i < nameItems.length; i++) {
      // Create copy of inJson
      Map<String,dynamic> jsonCopy = Map.from(inJson);
      // Replace main/name in jsonCopy with items from nameItems & mainItems
      jsonCopy['name'] = nameItems[i];
      jsonCopy['main'] = mainItems[i];
      // Add jsonCopy to jsonList
      jsonList.add(jsonCopy);
    }
  }

  if (style == 'ListModelStruct') {
    // Return list of JSONs with list of strings for Name & Main
    // First - Convert "name" and "main" into ListModelStruct using splitListStringToListList and pass nameItems/mainItems and inPartSep
    List<ListModelStruct> nameList = splitListStringToListList(nameItems, inPartSep, allowEmptyStrings);
    List<ListModelStruct> mainList = splitListStringToListList(mainItems, inPartSep, allowEmptyStrings);
  
    // Return = list of JSONs with list of strings for Name & Main
    for (int i = 0; i < nameList.length; i++) {
      // Create copy of inJson
      Map<String,dynamic> jsonCopy = Map.from(inJson);
      // Replace main/name in jsonCopy with items from nameList/mainList
      jsonCopy['name'] = nameList[i].list;
      jsonCopy['main'] = mainList[i].list;
      // Add jsonCopy to jsonList
      jsonList.add(jsonCopy);
    }
  }
return jsonList;
}

Any ideas (fixes or work arounds)?

Custom Action Won't Compile - no errors given