### 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
"""
    Thread that launches a process and then streams its output back to the main
    command. We use threads to avoid using select() and raw filehandles, and the
    complex logic that brings doing line buffering.
    """
    def __init__(self, parent, name: str, command: List[str], env: Dict[str, str]):
        super().__init__()
        self.parent = parent
        self.name = name
        self.command = command
        self.env = env
    def run(self):
        """Runs the actual process and captures it output to a queue"""
        self.process = subprocess.Popen(
            ["airflow"] + self.command,
            stdout=subprocess.PIPE,
            stderr=subprocess.STDOUT,
            env=self.env,
        )
        for line in self.process.stdout:
            self.parent.output_queue.append((self.name, line))
    def stop(self):
        """Call to stop this process (and thus this thread)"""
        self.process.terminate()
standalone = StandaloneCommand.entrypoint