Defer first update active buffer for conversation (#10564)

This fixes when the workspace is not actually available for a
`.read(cx)`.

Release Notes:

- Fix a panic when quoting a selection before the assistant panel has
been started

Co-authored-by: Conrad Irwin <conrad@zed.dev>
5037f466f6Kyle Kelley committed on 4/15/2024, 5:11:02 PM· committed by GitHubparentf28fde5
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