fix(tui): defer prompt.set() to prevent crash when using --prompt flag

[ryanrhughes]

What does this PR do?

Fixes a regression caused by #7702 that causes a crash when using the --prompt flag.

Fixes #7818 Fixes #7850

How did you verify your code works?

I ran tests with --prompt populated and without, as well as tested that you can still successfully fork from the first message so as to not cause any regression from the original PR.

[github-actions[bot]]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[markjaquith]

Regression was fixed in 3a30773874ccb92d085eddbab66d053c5d4e6326