Merge pull request #285 from jingtaozf/main

Check if the z-index returns an empty value safely.

source/clog-gui.lisp

@@ -559,7 +559,8 @@ The on-window-change clog-obj received is the new window"))
         (maphash (lambda (key value)
                    (declare (ignore key))
                    (setf new-order (z-index value))
-                   (when (>= new-order order)
+                   (when (and new-order
+                              (>= new-order order))
                      (setf order new-order)
                      (setf obj value)))
                  (windows app))))