patch 8.1.1441: popup window filter not yet implemented

Problem:    Popup window filter not yet implemented.
Solution:   Implement the popup filter.
diff --git a/src/window.c b/src/window.c
index 42689ee..b4ab11b 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4844,6 +4844,9 @@
 #ifdef FEAT_MENU
     remove_winbar(wp);
 #endif
+#ifdef FEAT_TEXT_PROP
+    free_callback(&wp->w_filter_cb);
+#endif
 
 #ifdef FEAT_SYN_HL
     vim_free(wp->w_p_cc_cols);