static List<String?> _lubanCompressList(List objects) {
var results = [];
objects.forEach((_o) {
results.add(_lubanCompress(_o));
});
return results as List<String?>;
}

The above method was causing that error: Had to change to the below and resolved the error

static List<String?> _lubanCompressList(List objects) {
var results = [];
objects.forEach((_o) {
results.add(_lubanCompress(_o));
});
return (results as List<dynamic?>).cast<String?>();
}