Scrapy - sending email returns "builtins.AttributeError: 'NoneType' object has no attribute 'bio_read'"

Question

I intend to send emails when a spider finnishes scraping. I have a MyStatsCollector class (placed in the root of the project) with a code like this:

class MyStatsCollector(StatsCollector):
    def _persist_stats(self, stats, spider):
       ...
       mailer = MailSender()
       mailer.send()

The code is functional, it sends out the email when spider is done.

However, the code is also return an error:

Traceback (most recent call last):
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/python/log.py", line 96, in callWithLogger
    return callWithContext({"system": lp}, func, *args, **kw)
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/python/log.py", line 80, in callWithContext
    return context.call({ILogContext: newCtx}, func, *args, **kw)
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/python/context.py", line 117, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/python/context.py", line 82, in callWithContext
    return func(*args, **kw)
--- <exception caught here> ---
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/internet/asyncioreactor.py", line 138, in _readOrWrite
    why = method()
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/internet/tcp.py", line 248, in doRead
    return self._dataReceived(data)
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/internet/tcp.py", line 253, in _dataReceived
    rval = self.protocol.dataReceived(data)
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/protocols/tls.py", line 329, in dataReceived
    self._flushReceiveBIO()
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/protocols/tls.py", line 300, in _flushReceiveBIO
    self._flushSendBIO()
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/protocols/tls.py", line 253, in _flushSendBIO
    bytes = self._tlsConnection.bio_read(2 ** 15)
builtins.AttributeError: 'NoneType' object has no attribute 'bio_read'


Traceback (most recent call last):
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/internet/asyncioreactor.py", line 138, in _readOrWrite
    why = method()
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/internet/tcp.py", line 248, in doRead
    return self._dataReceived(data)
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/internet/tcp.py", line 253, in _dataReceived
    rval = self.protocol.dataReceived(data)
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/protocols/tls.py", line 329, in dataReceived
    self._flushReceiveBIO()
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/protocols/tls.py", line 300, in _flushReceiveBIO
    self._flushSendBIO()
  File "/opt/homebrew/lib/python3.10/site-packages/twisted/protocols/tls.py", line 253, in _flushSendBIO
    bytes = self._tlsConnection.bio_read(2 ** 15)
AttributeError: 'NoneType' object has no attribute 'bio_read'

After some googling, I found this issue on Github where it is advised to implement sending emails with the async/await mechanism.

How can one do that? What parts do need to be modified?