Bcrypt with graphQL and nestJS

In NestJS and GraphQL while using Bcrypt I had to use it with promise like below

async getUser(email: String, password: String) {
    let user: User[] | User | null = await this.userModel.find({ email });
    user = user.length && user[0];

    if (user) {
      return bcrypt
        .compare(password, user.password)
        .then((res: any) => {
          if (res) {
            return { result: true, message: 'Logged in successfully.'};
          } else return { result: false, message: 'Incorrect Password.' };
        })
        .catch((err) => {
          return { result: false, message: err.message };
        });
    } else {
      return { result: false, message: 'User not found. Try Sign-up!' };
    }
  }

otherwise, if I was using it in a normal fashion like below

bcrypt.compare(myPlaintextPassword, hash, function(err, result) {
    // result == true
});

it was throwing an error like
"Cannot return null for non-nullable field Query.getUser."

Does anyone know why this behavior?