If Xwayland has crashed, the Workspace will block stacking order updates
and start destroying all X11 clients.

Once stacking order updates are unblocked, the Workspace will mark the X
stacking order as dirty and create a new Xcb::Tree object.

We don't want to create that Xcb::Tree object because accessing it
after the XCB connection has been shut down will lead to a crash.

BUG: 427688
FIXED-IN: 5.20.1


(cherry picked from commit 2093820a)