### Apache Airflow version

2.2.1 (latest released)

### Operating System

Ubuntu (16.04)

### Versions of Apache Airflow Providers

apache-airflow-providers-celery==2.1.0
apache-airflow-providers-ftp==2.0.1
apache-airflow-providers-http==2.0.1
apache-airflow-providers-imap==2.0.1
apache-airflow-providers-sqlite==2.0.1


### Deployment

Virtualenv installation

### Deployment details

_No response_

### What happened

Got an error while running `$ airflow standalone`.


Stacktrace:

 webserver | [2021-11-02 16:59:56 +0530] [15195] [INFO] Booting worker with pid: 15195
Traceback (most recent call last):
  File "/home/jonathan/programs/venv/bin/airflow", line 8, in <module>
    sys.exit(main())
  File "/home/jonathan/programs/venv/lib/python3.6/site-packages/airflow/__main__.py", line 48, in main
    args.func(args)
  File "/home/jonathan/programs/venv/lib/python3.6/site-packages/airflow/cli/cli_parser.py", line 48, in command
    return func(*args, **kwargs)
  File "/home/jonathan/programs/venv/lib/python3.6/site-packages/airflow/cli/commands/standalone_command.py", line 48, in entrypoint
    StandaloneCommand().run()
  File "/home/jonathan/programs/venv/lib/python3.6/site-packages/airflow/cli/commands/standalone_command.py", line 95, in run
    if not self.ready_time and self.is_ready():
  File "/home/jonathan/programs/venv/lib/python3.6/site-packages/airflow/cli/commands/standalone_command.py", line 209, in is_ready
    return self.port_open(8080) and self.job_running(SchedulerJob) and self.job_running(TriggererJob)
  File "/home/jonathan/programs/venv/lib/python3.6/site-packages/airflow/cli/commands/standalone_command.py", line 231, in job_running
    return job.most_recent_job().is_alive()
AttributeError: 'NoneType' object has no attribute 'is_alive'


### What you expected to happen

I expect the server to start without any errors or stacktraces.

### How to reproduce

Follow the installation instructions in the docs and then run `$ airflow standalone`.

### Anything else

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

airflow/cli/commands/standalone_command.py
#
#
import logging
import os
import random
import socket
import subprocess
import threading
import time
from collections import deque
from typing import Dict, List
from termcolor import colored
from airflow.configuration import AIRFLOW_HOME, conf
from airflow.executors import executor_constants
from airflow.jobs.scheduler_job import SchedulerJob
from airflow.jobs.triggerer_job import TriggererJob
from airflow.utils import db
from airflow.www.app import cached_app
class StandaloneCommand: