fix: do not set BidiRpc to None when closing it

[Linchin]

For more context see comment #638 (comment).

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #638 🦕

[shollyman]

Any tests that would be helpful to validate this, or are they already present?

[Linchin]

I don't think there's a test covering this case. I can create another PR adding it.