### 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
"""
        Prints an error message to the console (this is the same as
        print_output but with the text red)
        """
        self.print_output(name, colored(output, "red"))
    def calculate_env(self):
        """
        Works out the environment variables needed to run subprocesses.
        We override some settings as part of being standalone.
        """
        env = dict(os.environ)
        
        if conf.get("core", "executor") not in [
            executor_constants.LOCAL_EXECUTOR,
            executor_constants.SEQUENTIAL_EXECUTOR,
        ]:
            if "sqlite" in conf.get("core", "sql_alchemy_conn"):
                self.print_output("standalone", "Forcing executor to SequentialExecutor")
                env["AIRFLOW__CORE__EXECUTOR"] = executor_constants.SEQUENTIAL_EXECUTOR
            else:
                self.print_output("standalone", "Forcing executor to LocalExecutor")
                env["AIRFLOW__CORE__EXECUTOR"] = executor_constants.LOCAL_EXECUTOR
        return env
    def initialize_database(self):