patch 8.1.1446: popup window callback not implemented yet

Problem:    Popup window callback not implemented yet.
Solution:   Implement the callback.
diff --git a/src/window.c b/src/window.c
index e15b311..f4ef983 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4845,6 +4845,7 @@
     remove_winbar(wp);
 #endif
 #ifdef FEAT_TEXT_PROP
+    free_callback(&wp->w_close_cb);
     free_callback(&wp->w_filter_cb);
     for (i = 0; i < 4; ++i)
 	VIM_CLEAR(wp->w_border_highlight[i]);