fix(lsp): handle invalid buffer when resolving lenses

[tomtomjhj]

Problem: While waiting for codelens response for a buffer, the buffer may have been deleted. This results in invalid buffer error when resolving the lenses.

Problem: Ensure validity of the buffer before resolving.

[tomtomjhj]

Can someone merge this?

[tomtomjhj]

This shouldn't be necessary as client.request now handles invalid buffers.