[Bug] [Lean] Clicking into the file quickly breaks the proof view

[pimotte]

How can we reproduce this issue?

Example:

1. Start the extension with a Lean file open
2. Immediately click a proof area.
3. Observe the infoview being stuck at Waiting for Lean server to start...

What was supposed to happen?

It should update to show the goals.

(Probably some sort of race condition)