[BUG] Panic when opening empty menubar subtree

[DvvCz]

Describe the bug
When creating a menubar with an empty subtree, panics when you select it in the menu.
Dealing with this as I want to create a dynamic menubar to select a file, although maybe this isn't the best approach.

To Reproduce

let mut siv = cursive::default();
siv.menubar().add_subtree("foo", Tree::new());
siv.add_global_callback(Key::Esc, |s| s.select_menubar());
siv.run();
// Then press escape, go to foo and press enter.

Expected behavior
Expected to not panic

Environment

• OS: Linux (Nobara 39)
• Backend used: ncurses
• Current locale: en_US
• Cursive version: 0.20.0
• Nightly rust in debug build

Additional context
Assuming index OOB error "len is 0 but index is 0":

cursive/cursive-core/src/views/menu_popup.rs

Line 331 in 0a95c82

let item = &s.menu.children[i];

As panic points to around there (line 335 specifically).

[gyscos]

Thanks for the report!

Indeed, not panicking sounds like a better behavior :)