DatabaseException(FOREIGN KEY constraint failed (code 787)) sql 'INSERT OR FAIL INTO - sql

Question

I'm having problem with foreign key while adding a new habit and inserting it's data in db and I don't really know why, because everything seems right. the error is:
[ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: DatabaseException(FOREIGN KEY constraint failed (code 787)) sql 'INSERT OR FAIL INTO Habits (userId, text, emoji, period, startPeriod) VALUES (?, ?, ?, ?, NULL)' args [1, habit, ☺, [1,2,3,4,5,6,7]]
error accrued here:
class DataService {...
late final Database db;
Future<void> addHabit(Habit newHabit) async {
final newHabitId = await HabitsDb.createHabit(db, newHabit);//error accrued here
final days = (await HabitsDb.getAllDays(db)).map((e) => Day.fromDb(e, []));
for (final day in days) {
if (newHabit.startPeriod != null &&
newHabit.startPeriod!.compareTo(day.date) > 0) {
continue;
}
if (newHabit.period.contains(day.date.weekday)) {
await HabitsDb.createHabitDay(
db, day.date.millisecondsSinceEpoch, newHabitId);
}
}
await _reloadDaysInCache();
_loadHabits();
} ...}
then:
class HabitsDb{...
static Future<int> createHabit(Database db, Habit habit) async {
return db.insert(HabitsTable.tableName, await habit.toDb(),
conflictAlgorithm: ConflictAlgorithm.fail);
}...}
model:
class Habit {
late final int id;
late final int userId;
late String text;
late String emoji;
late final List<int> period;
late final DateTime? startPeriod;
...
Future<Map<String, dynamic>> toDb() async {
print(text);
print(userId);
return {
//HabitsTable.id: id,
HabitsTable.userId: userId,
HabitsTable.text: text,
HabitsTable.emoji: emoji,
HabitsTable.period: jsonEncode(period),
HabitsTable.startPeriod: startPeriod?.millisecondsSinceEpoch
};
}...}
creating table:
class HabitsTable {
static const String tableName = 'Habits';
static const String id = 'id';
static const String userId = 'userId';
static const String text = 'text';
static const String emoji = 'emoji';
static const String period = 'period';
static const String startPeriod = 'startPeriod';
static const String createQuery = '''
CREATE TABLE IF NOT EXISTS $tableName (
$id INTEGER PRIMARY KEY AUTOINCREMENT,
$userId INTEGER NOT NULL,
$text TEXT NOT NULL,
$emoji TEXT NOT NULL,
$period TEXT NOT NULL,
$startPeriod INTEGER,
FOREIGN KEY($userId) REFERENCES ${UsersTable.tableName}(id));''';
}
if more code is needed you can check https://github.com/sarasoltan/habit_tracker
plz help if you know how to solve this error.