Fix potential null reference when disposing of the TCP Manager

[nathanwoctopusdeploy]

Background

Related to https://github.com/OctopusDeploy/Issues/issues/8266

How to review this PR

Quality :heavy_check_mark:

Pre-requisites

• [ ] I have read How we use GitHub Issues for help deciding when and where it's appropriate to make an issue.
• [ ] I have considered informing or consulting the right people, according to the ownership map.
• [ ] I have considered appropriate testing for my change.

[nathanwoctopusdeploy]

LGTM. I saw that error, and was about to put a card on the board 😁 Nice one.

I made the issue so thought I should fix it 😆