Debugger raises an exception in VSCode when it stops at exception

[ono-max]

Your environment

$ ruby -v
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin20]

Describe the bug

$ exe/rdbg ../target.rb --open=vscode
DEBUGGER: Debugger can attach via UNIX domain socket (/var/folders/kv/w1k6nh1x5fl7vx47b2pd005w0000gn/T/ruby-debug-sock-501/ruby-debug-naotto-18645)
DEBUGGER: wait for debugger connection...
Launching: code /var/folders/kv/w1k6nh1x5fl7vx47b2pd005w0000gn/T/ruby-debug-vscode-20211212-18645-1yffto3/ /var/folders/kv/w1k6nh1x5fl7vx47b2pd005w0000gn/T/ruby-debug-vscode-20211212-18645-1yffto3/README.rb
DEBUGGER: Connected.
["DEBUGGER Exception: /Users/naotto/workspace/debug/lib/debug/thread_client.rb:982",
 #<NoMethodError: undefined method `local_variable_defined?' for nil:NilClass>,
 ["/Users/naotto/workspace/debug/lib/debug/thread_client.rb:443:in `block in special_local_variables'",
  "/Users/naotto/workspace/debug/lib/debug/thread_client.rb:441:in `each'",
  "/Users/naotto/workspace/debug/lib/debug/thread_client.rb:441:in `special_local_variables'",
  "/Users/naotto/workspace/debug/lib/debug/server_dap.rb:583:in `process_dap'",
  "/Users/naotto/workspace/debug/lib/debug/thread_client.rb:971:in `wait_next_action_'",
  "/Users/naotto/workspace/debug/lib/debug/thread_client.rb:672:in `wait_next_action'",
  "/Users/naotto/workspace/debug/lib/debug/thread_client.rb:279:in `suspend'",
  "/Users/naotto/workspace/debug/lib/debug/thread_client.rb:217:in `on_breakpoint'",
  "/Users/naotto/workspace/debug/lib/debug/breakpoint.rb:63:in `suspend'",
  "/Users/naotto/workspace/debug/lib/debug/breakpoint.rb:299:in `block in setup'",
  "../target.rb:8:in `/'",
  "../target.rb:8:in `bar'",
  "../target.rb:2:in `foo'",
  "../target.rb:11:in `<main>'"]]
DEBUGGER: **Disconnected.**

To Reproduce

1. Check rescue any exception
2. Push "Continue" button.
3. It raise an exception when I try to see "Variables" tab.

def foo
  bar
rescue ZeroDivisionError
  raise
end

def bar
  1/0
end

foo

Expected behavior
It should not raise an exception.

Additional context
This bug also occurs in Chrome.

[st0012]

It's because the new special locals feature currently doesn't work in C frames. #424 should fix the issue.

[ko1]

#441 fix it.