How to fix Flutter NosuchmethodError?

I'm getting this error:

NosuchmethodError: Class 'List' has no instance setter 'state='. Recevier: Instance(length:0) of ' _GrowableList' Tried calling: state=Instance(lenght:15) of'_GrowableList'

and I don't know where is the problem I can't see any problem in the code.

import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:quiz2/const/const.dart';
import 'package:sqflite/sqflite.dart';

class Category {
              int ID;
              String name, image;        
                          
              Map<String, dynamic> toMap() {
                var map = <String, dynamic> {
                  columnMainCategoryId: ID,
                  columnCategoryName: name,
                  columnCategoryImage: image
                };
                return map;
              }
              
              Category();
            
              Category.fromMap(Map<String,dynamic> map) {
                ID = map[columnMainCategoryId];
                name = map[columnCategoryName];
                image = map[columnCategoryImage];
              }
           }
                
           class CategoryProvider {
                   
                   Future<Category> getCategoryById(Database db, int id)async {
                    var maps = await db.query(tableCategoryName, 
                    columns: [
                      columnMainCategoryId,
                      columnCategoryName,
                      columnCategoryImage
                
                     ], where: "$columnMainCategoryId=?",
                     whereArgs: [id]);
                
                     if(maps.length > 0)
                     return Category.fromMap(maps.first);
                     return null;
                   }
                
                   Future<List<Category>> getCategories(Database db) async{
                    var maps = await db.query(tableCategoryName, 
                    columns: [
                      columnMainCategoryId,
                      columnCategoryName,
                      columnCategoryImage
                    ]);
                
                     if(maps.length > 0) return maps.map((category) => Category.fromMap(category)).toList();
                     return null;
                   }
                  }
                
                  class CategoryList extends StateNotifier<List<Category>>{
                  
                  CategoryList(List<Category> state):super(state ?? []);
                   
                   void addAll(List<Category> category)
                   {
                     state.addAll(category);
                   }
                    
                    void add(Category category){
                      state = [
                        ...state,
                        category,
                      ];
                    }
                
                }

file const.dart

final db_name = "quiz.db";

final columnMainCategoryId = "ID";
final columnCategoryName = "Name";
final columnCategoryImage = "Image";
final tableCategoryName = "Category";

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:quiz2/screens/home_page.dart';

void main() {
  runApp(ProviderScope(child:MyApp()));
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      routes: {
        "/homePage": (context) => MyCategoryPage(title: "My Quiz",)
      },
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Topics'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  @override
  void initState() { 
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
      Navigator.of(context).pop();
      Navigator.pushNamed(context, "/homePage");
    });
  }
  

  @override
  Widget build(BuildContext context) {
     return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
       // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

screens/home_page.dart

import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:auto_size_text/auto_size_text.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:quiz2/const/state.dart';
import 'package:quiz2/database/category_provider.dart';
import 'package:quiz2/database/db_helper.dart';



class MyCategoryPage extends StatefulWidget {
 MyCategoryPage({Key key, this.title}):super(key: key);
 final String title;

  @override
  _MyCategoryPageState createState() => _MyCategoryPageState();
}


class _MyCategoryPageState extends State<MyCategoryPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: FutureBuilder<List<Category>>(
        future: getCategories(),
                builder: (context, snapshot){
                 if(snapshot.hasError)
                  return Center(child: Text('${snapshot.error}'),);
                 else if(snapshot.hasData)
                 {
                    Category category = new Category();
                    category.ID = -1;
                    category.name = "Exam";
                    snapshot.data.add(category); 
        
                    return GridView.count(
                      crossAxisCount: 2,
                      childAspectRatio: 1.0,
                      padding: const EdgeInsets.all(4.0),
                      mainAxisSpacing: 4.0,
                      crossAxisSpacing: 4.0,
                      children: snapshot.data.map((category){ 
                        return GestureDetector(child: Card(
                          elevation: 2,
                          color: category.ID == -1 ? Colors.green : Colors.white,
                          child: Column(
                            crossAxisAlignment: CrossAxisAlignment.center,
                            mainAxisAlignment: MainAxisAlignment.center,
                            children: [
                               Center(
                                 child: AutoSizeText(
                                   "${category.name}",
                                   style: TextStyle(
                                     color: category.ID == -1 ? Colors.white : Colors.black,
                                     fontWeight: FontWeight.bold
                                   ),
                                   maxLines: 1,
                                   textAlign: TextAlign.center,
                                 )
                              ,)
                            ],
                          ),
                          ),);
                      }).toList(),
                    );
                 }
                 else return Center(child: CircularProgressIndicator(),);
                },
              )
            );
          }
        
        Future<List<Category>>  getCategories() async {
           var db = await copyDB();
           var result = await CategoryProvider().getCategories(db);
           context.read(categoryListProvider).state = result;
           return result; 
        }
  }

db_helper.dart

import 'dart:io';

import 'package:flutter/services.dart';
import 'package:quiz2/const/const.dart';
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';

Future<Database> copyDB() async{
  var dbPath = await getDatabasesPath();
  var path = join(dbPath, db_name);

  var exists = await databaseExists(path);
  if(!exists){
    try{
      await Directory(dirname(path)).create(recursive:true);
     } catch(_){

    }
    
    ByteData data = await rootBundle.load(join("assets/db",db_name));
    List<int> bytes = data.buffer.asUint8List(data.offsetInBytes,data.lengthInBytes);
    await File(path).writeAsBytes(bytes, flush:true);
  }else{
    print("DB already exists!");
  }
  return await openDatabase(path, readOnly: true);
}

state.dart

import 'package:flutter_riverpod/all.dart';
import 'package:quiz2/database/category_provider.dart';

final categoryListProvider = StateNotifierProvider((ref) => new CategoryList([]));

NosuchmethodError: Class 'List' has no instance setter 'state='. Recevier: Instance(length:0) of ' _GrowableList' Tried calling: state=Instance(lenght:15) of'_GrowableList'

In your code, you have

Future<List<Category>>  getCategories() async {
   var db = await copyDB();
   var result = await CategoryProvider().getCategories(db);
   context.read(categoryListProvider).state = result;
   return result; 
}

Here, your context.read(categoryListProvider) is actually of type List.
That's why, when you are calling it like that, it means that you are calling,

[/* some kind of list */].state = result

But, the List class doesn't have a property called state but you are trying to modify it. That's why the error is coming.

Comment out that line and check.