Handle when transport has been garbage collected.

[Siecje]

Checklist

• I have read the contribution guidelines.
• I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • I edited the corresponding document in the devdoc folder and added or modified requirements.

Reference/Link to the issue solved with this PR (if any)

#1192

Description of the problem

this can be None if the object has already been garbage collected.

Description of the solution

Check if this is None and log as in on_disconnect.