Extracting Values from JSON Object (not API)

I'm encountering an error ('type 'Null' is not a subtype of type 'List<dynamic>') when trying to extract an array of integers from a JSON object for use in a bar chart. I assume that this means the list contains null values. I have a custom function to aggregate my data:

dynamic calculateFrequenciesForProject(
  CategoriesStruct metrics,
  List<dynamic> measurements,
) {
  /// MODIFY CODE ONLY BELOW THIS LINE

  // Initialize frequency map for each question
  Map<String, List<int>> frequencyMap = {};
  metrics.toMap().forEach((question, options) {
    frequencyMap[question] = List<int>.filled(options!.length, 0);
  });

  // Loop through each measurement
  for (var measurement in measurements) {
    var currentMeasurement = measurement;
    // If measurement is a JSON string, decode it to a map
    if (currentMeasurement is String) {
      currentMeasurement = jsonDecode(currentMeasurement);
    }

    // Loop through each question
    metrics.toMap().forEach((question, options) {
      var answer = currentMeasurement[question];

      // Increment frequency if answer exists and is valid
      if (answer != null) {
        var index = options!.indexOf(answer);
        if (index != -1) {
          frequencyMap[question]![index]++;
        }
      }
    });
  }

  // Convert the frequency map to a single JSON object
  Map<String, dynamic> frequencyObject = {};
  frequencyMap.forEach((question, frequencies) {
    frequencyObject[question] = frequencies;
  });

  // Encode the frequency object to JSON string
  String jsonString = jsonEncode(frequencyObject);

  // Return the JSON string
  return jsonString;

  /// MODIFY CODE ONLY ABOVE THIS LINE
}

My custom function appears to be working correctly. When used with a Text Widget, it returns the expected JSON object.

Now when I try to use JSON Path in the bar chart values e.g. $.measure or $.measure[0:] the array seems to be empty and I receive the error message as shown above.

Is my approach correct or are there alternatives?

I appreciate any help!