Panic due to assert_by proof action

[chanheec]

Reported by @jaylorch

First, I opened https://github.com/verus-lang/verus-systems-code/tree/main/ironfleet-comparison/ironsht. Then, I opened environment_t.rs and added a trivial bug:

proof fn testit()
{
    assert(false);
}

The server window shows the following crash:

thread 'Worker' panicked at 'called `Option::unwrap()` on a `None` value', crates/syntax/src/ast/edit_in_place.rs:607:58
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:64:14
   2: core::panicking::panic
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:111:5
   3: syntax::ast::edit_in_place::<impl syntax::ast::generated::nodes::AssertExpr>::make_by_keyword
   4: ide_assists::handlers::assert_by::assert_by
   5: ide_assists::assists
   6: std::panicking::try
   7: ide::Analysis::assists_with_fixes
   8: rust_analyzer::handlers::handle_code_action
   9: std::panicking::try
  10: <F as threadpool::FnBox>::call_box
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.