inserts an event into the default calendar using the Calendar API

It seems I can't insert an event into the default calendar on my device using react-native and expo sdk. I tried this:

import * as Permissions from 'expo-permissions';
import * as Calendar from 'expo-calendar';
import { Platform } from 'react-native';
...

async obtainDefaultCalendarId() {
    let calendar = null;
    if (Platform.OS === 'ios') {
      calendar = await Calendar.getDefaultCalendarAsync();
    } else {
      const calendars = await Calendar.getCalendarsAsync();
      calendar = calendars
        ? calendars.find((cal) => cal.isPrimary) || calendars[0]
        : null;
    }
    return calendar ? calendar.id.toString() : null;
  }

  async obtainCalendarPermission() {
    let permission = await Permissions.getAsync(Permissions.CALENDAR);
    if (permission.status !== 'granted') {
      permission = await Permissions.askAsync(Permissions.CALENDAR);
      if (permission.status !== 'granted') {
        Alert.alert('Permission not granted to access the calendar');
      }
    }
    return permission;
  }

  async addReservationToCalendar(date) {
    await this.obtainCalendarPermission();
    CalendarId = this.obtainDefaultCalendarId();
    const startDate = new Date(Date.parse(date));
    const endDate = new Date(Date.parse(date) + 2 * 60 * 60 * 1000);
    Calendar.createEventAsync(CalendarId, {
      title: 'Con Fusion Table Reservation',
      location:
        '121, Clear Water Bay Road, Clear Water Bay, Kowloon, Hong Kong',
      startDate: startDate,
      endDate: endDate,
      timeZone: 'Asia/Hong_Kong',
    });
    Alert.alert('Reservation has been added to your calendar');
  }
...

It asked for the permission but didn't add an event to my calendar. Before I see any error in my screen the expo client app is crushed.

I didn't get where the error occurs. When I console.log(CalendarId), I got this,

Promise {
  "_40": 0,
  "_55": null,
  "_65": 0,
  "_72": null,
}

I heartily thank if anyone helps me to figure out this.