patch 9.0.0076: no test for what patch 8.1.1424 fixes

Problem:    No test for what patch 8.1.1424 fixes.
Solution:   Add a test. (closes #10789)
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index 5fb7e82..af14167 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -984,6 +984,25 @@
   endtry
 endfunc
 
+" This used to crash before patch 8.1.1424
+func Test_popup_delete_when_shown()
+  CheckFeature menu
+  CheckNotGui
+
+  func Func()
+    popup Foo
+    return "\<Ignore>"
+  endfunc
+
+  nmenu Foo.Bar :
+  nnoremap <expr> <F2> Func()
+  call feedkeys("\<F2>\<F2>\<Esc>", 'xt')
+
+  delfunc Func
+  nunmenu Foo.Bar
+  nunmap <F2>
+endfunc
+
 func Test_popup_complete_info_01()
   new
   inoremap <buffer><F5> <C-R>=complete_info().mode<CR>