Support null-safety

example/example.dart

@@ -33,27 +33,27 @@ final config = MigrationConfig(
     initializationScript: initialScript, migrationScripts: migrations);
 
 class DatabaseRepository {
-  static Database _database;
-  String path;
+  static Database? _database;
+  String? path;
 
   Future<Database> get database async {
     if (_database != null) {
-      return _database;
+      return _database!;
     }
 
     _database = await _open();
-    return _database;
+    return _database!;
   }
 
   Future<Database> _open() async {
-    final databasesPath = await getDatabasesPath();
+    final databasesPath = await (getDatabasesPath() as FutureOr<String>);
     final path = join(databasesPath, 'test.db');
 
     return await openDatabaseWithMigration(path, config);
   }
 
-  Future<Map> findById(int id) async {
-    final db = await database;
+  Future<Map?> findById(int id) async {
+    final db = await (database as FutureOr<Database>);
     List<Map> maps = await db.query('_task',
         columns: ['id', 'description'], where: 'id = ?', whereArgs: [id]);
     if (maps.length > 0) {

lib/src/migration_config.dart

@@ -13,9 +13,5 @@ class MigrationConfig {
   final List<String> initializationScript;
   final List<String> migrationScripts;
 
-  MigrationConfig({this.initializationScript, this.migrationScripts})
-      : assert(initializationScript != null,
-            'The initializationScript cannot be null.'),
-        assert(
-            migrationScripts != null, 'The migrationScripts cannot be null.');
+  MigrationConfig({required this.initializationScript, required this.migrationScripts});
 }

lib/src/migrator.dart

@@ -14,7 +14,7 @@ class Migrator {
 
   Migrator(this.config);
 
-  Future<void> executeInitialization(Database db, int version) async {
+  Future<void> executeInitialization(DatabaseExecutor db, int version) async {
     for (String script in config.initializationScript) {
       await db.execute(script);
     }
@@ -25,7 +25,7 @@ class Migrator {
   }
 
   Future<void> executeMigration(
-      Database db, int oldVersion, int newVersion) async {
+      DatabaseExecutor db, int oldVersion, int newVersion) async {
     assert(oldVersion < newVersion,
         'The newVersion($newVersion) should always be greater than the oldVersion($oldVersion).');
     assert(config.migrationScripts.length == newVersion - 1,