Replace unwrap() with a check and an error message

I just crashed here somehow.

src/handlers/xdg_shell.rs

@@ -158,11 +158,18 @@ impl XdgShellHandler for State {
             return;
         }
 
-        let (window, output) = self
+        let win_out = self
             .niri
             .layout
-            .find_window_and_output(surface.wl_surface())
-            .unwrap();
+            .find_window_and_output(surface.wl_surface());
+
+        let Some((window, output)) = win_out else {
+            // I have no idea how this can happen, but I saw it happen once, in a weird interaction
+            // involving laptop going to sleep and resuming.
+            error!("toplevel missing from both unmapped_windows and layout");
+            return;
+        };
+
         self.niri.layout.remove_window(&window);
         self.niri.queue_redraw(output);
     }