webdev returns an internal server error when setting breakpoints on file path script uris

webdev returns an internal server error when setting breakpoints on file path script uris. Note that this isn't an issue for IntelliJ - I'm updating our code base to no longer try and set breakpoints by file path (just via dart:, package:, and file: uris). But I noticed that package:dwds throws an exception here - it should probably fail more gracefully.

```
==> {"jsonrpc":"2.0","id":"43","method":"addBreakpointWithScriptUri","params":{"isolateId":"1","scriptUri":"/Users/devoncarew/IdeaProjects/foo_flutter/lib/main.dart","line":9}}
```

```
<== {"jsonrpc":"2.0","error":{"code":-32603,"message":
"NoSuchMethodError: The getter 'id' was called on null.
Receiver: null
Tried calling: id
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)\n
#1 ChromeProxyService.addBreakpointWithScriptUri (package:dwds/src/services/chrome_proxy_service.dart:223:39)\n\n
#2 VmServerConnection._delegateRequest (package:vm_service/vm_service.dart:932:51)\n\n
#3 _rootRunUnary (dart:async/zone.dart:1132:38)\n
#4 _CustomZone.runUnary (dart:async/zone.dart:1029:19)\n
#5 _CustomZone.runUnaryGuarded (dart:async/zone.dart:931:7)\n
#6 _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)\n
#7 _BufferingStreamSubscription._add (dart:async/stream_impl.dart:263:7)\n
#8 _ForwardingStreamSubscription._add (dart:async/stream_pipe.dart:132:11)\n
#9 _MapStream._handleData (dart:async/stream_pipe.dart:232:10)\n
#10 _ForwardingStreamSubscription._handleData (dart:async/stream_pipe.dart:164:13)\n
...
```

Start at package dwds/src/services/chrome_proxy_service.dart:223 and reproduce the addBreakpointWithScriptUri request with the file-path URI shown. Trace how the script lookup handles a missing result, then verify the request returns a graceful response or error instead of NoSuchMethodError.

Written by the indexing model from the issue text.