diff --git a/src/package.lisp b/src/package.lisp
index 9f5b028..e4ac560 100644
--- a/src/package.lisp
+++ b/src/package.lisp
@@ -2081,6 +2081,8 @@
    :calculate
    :draw
    :aabb
+   :child-window-exists-p
+   :remove-child-window
    :remove-intersecting-window
    :draw-all
    :refresh-config-all
diff --git a/src/program-events.lisp b/src/program-events.lisp
index e6fe7f9..f2a21a8 100644
--- a/src/program-events.lisp
+++ b/src/program-events.lisp
@@ -429,10 +429,9 @@
 
 (defmethod process-event ((object remove-notify-user-event))
   (let ((win (payload object)))
-    (when (mtree:find-child-if specials:*main-window*
-                               (lambda (node) (eq node win)))
+    (when (windows:child-window-exists-p specials:*main-window* win)
       (windows:win-close win)
-      (mtree:remove-child specials:*main-window* win))))
+      (windows:remove-child-window specials:*main-window* win))))
 
 (defclass change-window-title-event (program-event)
   ((window
diff --git a/src/windows.lisp b/src/windows.lisp
index 6ac5bd3..2148f56 100644
--- a/src/windows.lisp
+++ b/src/windows.lisp
@@ -283,6 +283,10 @@ height, position and so on)"
 
 (defgeneric aabb (object))
 
+(defgeneric child-window-exists-p (object child-window))
+
+(defgeneric remove-child-window  (object child-window))
+
 (defmethod refresh-config (object)
   object)
 
@@ -357,6 +361,13 @@ height, position and so on)"
         (h (win-height object)))
     (2d-utils:make-iaabb2 x y (1- (+ x w)) (1- (+ y h)))))
 
+(defmethod child-window-exists-p ((object wrapper-window) child-window)
+  (mtree:find-child-if object
+                       (lambda (node) (eq node child-window))))
+
+(defmethod remove-child-window ((object wrapper-window) child-window)
+  (mtree:remove-child object child-window))
+
 (defun remove-intersecting-window (&key (discarded-window-type '()))
   (labels ((copy-subwindows-stack ()
              (let ((copy '()))