Image cache updates race with script thread destruction

[jdm]

│ Shutting down the Constellation after generating an output file or exit flag specified
  │ thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: ()', ../src/libcore/result.rs:741
  │ stack backtrace:
  │    1:     0x5598639ff160 - sys::backtrace::tracing::imp::write::ha5f3f0d9df3eccc0xXt
  │    2:     0x559863a02045 - panicking::log_panic::_<closure>::closure.40991
  │    3:     0x559863a01ac0 - panicking::log_panic::hb6d5879621a98ec062x
  │    4:     0x5598639eadc3 - sys_common::unwind::begin_unwind_inner::h56feb86a7746f2474Ps
  │    5:     0x5598639eb478 - sys_common::unwind::begin_unwind_fmt::hbd638ea70768c304aPs
  │    6:     0x5598639fe7c1 - rust_begin_unwind
  │    7:     0x559863a2f1df - panicking::panic_fmt::h761c3d51c4df5175ZFK
  │    8:     0x5598628f68aa - dom::htmlimageelement::_<impl>::update_image::_<closure>::closure.147309
  │    9:     0x559863025299 - router::_<impl>::new::_<closure>::closure.17983
  │   10:     0x559863023dba - sys_common::unwind::try::try_fn::try_fn::h15085486313241966974
  │   11:     0x5598639fe608 - __rust_try
  │   12:     0x5598639fa9db - sys_common::unwind::try::inner_try::hb614b0fa85d9fb24CMs
  │   13:     0x559863027e1c - boxed::_<impl>::call_box::call_box::h3618774687057863163
  │   14:     0x559863a009a3 - sys::thread::_<impl>::new::thread_start::h73d591f2e83d6559I5w
  │   15:     0x7f6c83f83181 - start_thread
  │   16:     0x7f6c8479d47c - __clone
  │   17:                0x0 - <unknown>