ash: fix a crash related to floating windows and close all

When a desk that contains a floating window is closed (close all), and
the floating window on the about-to-be-closed desk gains activation,
there was a chance that we would try to activate the closed desk. This
CL is a small fix to that issue.

BUG=b:369054655

Change-Id: I4e9633f2941a107a06e981b7f21043a0eb3c7784
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6451453
Commit-Queue: Daniel Andersson <dandersson@chromium.org>
Reviewed-by: Xiaodan Zhu <zxdan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1446091}