floatwin: show error if window is closed immediately #11476

Autocmds may close window while it is being entered, then
win_set_minimal_style(wp) operates on an invalid pointer.

We could silently ignore this instead, but it is unlikely to be
intentional, so it is more useful to show an error.

fix #11383

src/nvim/api/vim.c

@@ -1096,6 +1096,10 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dictionary config,
   if (enter) {
     win_enter(wp, false);
   }
+  if (!win_valid(wp)) {
+    api_set_error(err, kErrorTypeException, "Window was closed immediately");
+    return 0;
+  }
   if (buffer > 0) {
     nvim_win_set_buf(wp->handle, buffer, err);
   }