Uncaught Error: TypeError: null: type 'JSNull' is not a subtype of type 'List<dynamic>' - flutter

Question

I'm looking for a way to copy the values in the category_name field to a list without using the add operator. The idea behind not using the add operator is to prevent the existing data from remaining in the list. For example, if in a newer JSON data set, the values in category_name are [xyz, food] & [efg, food], then I intend to print [xyz, efg] instead of [Veg, Veg, NonVeg, XYZ, efg].
Please note that the JSON data currently assigned to duplicates is meant to be dynamic which is why it is important for me to find a solution in which I can only assign the values in category_name from the current JSON data set which is meant to change with API calls.
void main() {
List<dynamic> list = [];
Map<String, dynamic> duplicates = {
"status": "success",
"data": [
{
"product_id": 17,
"category_name": [
"Veg",
"Dinner"
],
"restaurant_name": "Mocambo",
"product_name": "Panner Makhani",
"product_description": "Mouth Smacking Creamy Indian Gravy.",
"product_image": "/assets/product/WOw4Rc03-02-08.jpg",
"product_selling_price": "240",
"product_status": "active",
"product_quantity": "50",
"product_rating": "",
"product_rating_count": "",
"product_sell_count": "0"
},
{
"product_id": 16,
"category_name": [
"Veg",
"Dinner"
],
"restaurant_name": "Mocambo",
"product_name": "Panner Makhani",
"product_description": "Mouth Smacking Creamy Indian Gravy.",
"product_image": "/assets/product/WOw4Rc03-02-08.jpg",
"product_selling_price": "240",
"product_status": "active",
"product_quantity": "50",
"product_rating": "",
"product_rating_count": "",
"product_sell_count": "0"
},
{
"product_id": 15,
"category_name": [
"NonVeg",
"Snacks"
],
"restaurant_name": "Mocambo",
"product_name": "Cheese Steak Burger",
"product_description": "Tasty steak burger oozing with the goodness of creamy cheese",
"product_image": "/assets/product/SBuZnx02-54-20.jpg",
"product_selling_price": "150",
"product_status": "active",
"product_quantity": "20",
"product_rating": "",
"product_rating_count": "",
"product_sell_count": "0"
}
]
};
list = duplicates['data'].forEach((value) => value["category_name"][0]);
print(list);
}
I tried doing list = duplicates['data'].forEach((value) => value["category_name"][0]); thinking that everytime the JSON data in duplicates changes, the previous values will get overwritten with new ones. But it doesn't really work that way because of the above error. Any help will be appreciated.