**Apache Airflow version**: 2.0.0rc2
**Environment**: Breeze with example dags, Python 3.8 postgres.
- **OS** (e.g. from /etc/os-release): Linux
- **Kernel** (e.g. `uname -a`): Breeze CI image
- **Install tools**: Breeze: 
- **Executor**: LocalExecutor
```
./breeze start-airflow --backend postgres --load-example-dags --load-default-connections --install-airflow-version 2.0.0rc2 --skip-mounting-local-sources --python 3.8
```

**What happened**:

When testing airflow logging I occasionally stumble upon "KeyError' from `dag_procesing.py`. I am not sure exactly when it happens. It's not always reproducible but it looks like it is when I restart scheduler and trigger 'example_bash_operator.py" it happens rather randomly (1/10 times more or less). It does not happen always when I triggere task manually. DAG gets correctly executed after triggering, but the log is there and warniing printed in the logs right after the DAG finishes execution.

The error I see in scheduler's logs:

```
[2020-12-13 19:35:33,752] {dagbag.py:440} INFO - Filling up the DagBag from /usr/local/lib/python3.8/site-packages/airflow/example_dags/example_bash_operator.py                                                                                                                                          
Running <TaskInstance: example_bash_operator.run_after_loop 2020-12-13T19:35:30.648020+00:00 [queued]> on host 6611da4b1a27
[2020-12-13 19:35:34,517] {dagrun.py:444} INFO - Marking run <DagRun example_bash_operator @ 2020-12-13 19:35:30.648020+00:00: manual__2020-12-13T19:35:30.648020+00:00, externally triggered: True> successful                                                                                           
[2020-12-13 19:35:34,523] {scheduler_job.py:1193} INFO - Executor reports execution of example_bash_operator.run_after_loop execution_date=2020-12-13 19:35:30.648020+00:00 exited with status success for try_number 1                                                                                   
Process ForkProcess-34:
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/usr/local/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.8/site-packages/airflow/utils/dag_processing.py", line 365, in _run_processor_manager
    processor_manager.start()
  File "/usr/local/lib/python3.8/site-packages/airflow/utils/dag_processing.py", line 596, in start
    return self._run_parsing_loop()
  File "/usr/local/lib/python3.8/site-packages/airflow/utils/dag_processing.py", line 659, in _run_parsing_loop
    self._processors.pop(processor.file_path)
KeyError: '/usr/local/lib/python3.8/site-packages/airflow/example_dags/example_bash_operator.py'
[2020-12-13 19:35:35,589] {dag_processing.py:396} WARNING - DagFileProcessorManager (PID=1029759) exited with exit code 1 - re-launching
```

**What you expected to happen**:

No error in logs.

**How to reproduce it**:

```
./breeze start-airflow --backend postgres --load-example-dags --load-default-connections --install-airflow-version 2.0.0rc2 --skip-mounting-local-sources --python 3.8
```

Login to the webserver, enable 'example_bash_operator", wait for it to execute. Trigger the example DAG several times (always wait for the end of execution. It happens randomly (for me around 1/10 tasks)


**Anything else we need to know**:

airflow/utils/dag_processing.py
"""
        Sends information about the callback to be executed by DagFileProcessor.
        :param request: Callback request to be executed.
        :type request: CallbackRequest
        """
        if not self._parent_signal_conn:
            raise ValueError("Process not started.")
        try:
            self._parent_signal_conn.send(request)
        except ConnectionError:
            
            
            pass
    def send_sla_callback_request_to_execute(self, full_filepath: str, dag_id: str) -> None: