called `Option::unwrap()` on a `None` value saving proposal after adding custom boundary

[nmbooker]

I launched 'ltn' with rugby-and-environs map loaded. Created a custom boundary, gave it a name, then tried to save my proposal straight away (so I didn't lose the work creating the boundary).
After typing the filename and clicking the Submit button (or was it labelled Save?), this traceback happened:

Traceback:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/runner/work/abstreet/abstreet/abstutil/src/collections.rs:148:51
stack backtrace:
   0: rust_begin_unwind
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/panicking.rs:142:14
   2: core::panicking::panic
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/panicking.rs:48:5
   3: abstutil::collections::Counter<T>::max_key
   4: ltn::save::PreserveState::switch_to_state
   5: <ltn::save::SaveDialog as widgetry::app_state::State<ltn::app::App>>::event
   6: <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
   7: widgetry::runner::State<A>::event
   8: widgetry::runner::run::{{closure}}
   9: winit::platform_impl::platform::x11::EventLoop<T>::drain_events::{{closure}}::{{closure}}
  10: winit::platform_impl::platform::x11::EventLoop<T>::drain_events::{{closure}}
  11: winit::platform_impl::platform::x11::event_processor::EventProcessor<T>::process_event
  12: winit::platform_impl::platform::x11::EventLoop<T>::run_return::single_iteration
  13: winit::platform_impl::platform::x11::EventLoop<T>::run
  14: winit::platform_impl::platform::EventLoop<T>::run
  15: winit::event_loop::EventLoop<T>::run
  16: widgetry::runner::run
  17: ltn::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
[2023-02-14T18:20:32Z INFO  abstio::io_native] Wrote data/player/camera_state/zz/oneshot/rugby-and-environs.json
[2023-02-14T18:20:32Z INFO  abstio::io_native] Wrote data/player/maps.json
[nick@jay abstreet_linux_v0_3_39]$ ^C

Here's a screenshot of the custom boundary I created:

Full output is here:
ltn-full-output.txt

[nmbooker]

More info on my system:

[nick@jay abstreet_linux_v0_3_39]$ uname -a
Linux jay 6.1.10-100.fc36.x86_64 #1 SMP PREEMPT_DYNAMIC Mon Feb  6 19:58:39 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

[dabreegster]

Hi, and thanks for the bug report! This is expected behavior right now; this custom boundary feature is new and has a bunch of limitations listed in #1044. I've unfortunately not had time to work through them yet. Maybe in the meantime, there should be a warning upfront that this tool is experimental?

For my own reference, the area you've drawn is near https://www.openstreetmap.org/#map=15/52.3753/-1.2488. You've made use of the tool for exactly one of the cases where it's intended -- your north boundary is the rail line, so adjusting a boundary block-by-block wouldn't work; it would insist on stretching to the nearest roads (Technology Drive and Butler's Leap?). After drawing this boundary, did the "Design LTN" tool work pretty reasonably for you?