xwayland: Avoid creating a tree query on crash
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