saved_desks: Fix crash in DeskMiniView when saving desk

DeskMiniView's constructor installs a focus predicate that may be called
after the desk that the mini view refers to has been destroyed. The
predicate was assuming that `desk_` is not null, but this doesn't always
hold. The mini view already tracks the deletion of its desk and sets
`desk_` to null when this happens, so we can simply check in the
predicate.

BUG=b:263159388

Change-Id: I21f3db7bb4c2902c469a5ad776167c82fe8d9a4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4116053
Commit-Queue: Daniel Andersson <dandersson@chromium.org>
Reviewed-by: Yongshun Liu <yongshun@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1085187}

ash/wm/desks/desk_mini_view.cc

@@ -126,7 +126,7 @@ DeskMiniView::DeskMiniView(DesksBarView* owner_bar,
             IsPointOnMiniView(
                 owner_bar_->last_dragged_item_screen_location())) ||
            desk_preview_->IsViewHighlighted() ||
-           (desk_->is_active() &&
+           (desk_ && desk_->is_active() &&
             !owner_bar_->overview_grid()->IsShowingSavedDeskLibrary());
   });
 

ash/wm/desks/desk_mini_view.h

@@ -160,9 +160,8 @@ class ASH_EXPORT DeskMiniView : public views::View,
   // The root window on which this mini_view is created.
   aura::Window* const root_window_;
 
-  // The associated desk. Can be null when the desk is deleted before this
-  // mini_view completes its removal animation. See comment above
-  // OnDeskRemoved().
+  // The associated desk. This can become null if the desk is deleted before the
+  // mini view is done. Desk deletion is monitored by `OnDeskDestroyed`.
   Desk* desk_;  // Not owned.
 
   // The view that shows a preview of the desk contents.