DevTools produces an error when opening in external tab on latest (master) SDK

[DanTup]

When launching in an external window this error occurs:

[5:03:05 PM] [VmService] [Error] [Flutter (iPhone 13)] Error handling 'callService' custom request: 500 Failed to launch browser: type 'Null' is not a subtype of type 'String'
#0      DevToolsServer.launchDevTools (package:dds/devtools_server.dart:507:11)
#1      MachineModeCommandHandler.registerLaunchDevToolsService.<anonymous closure> (package:dds/src/devtools/machine_mode_command_handler.dart:378:26)
#2      MachineModeCommandHandler.registerLaunchDevToolsService.<anonymous closure> (package:dds/src/devtools/machine_mode_command_handler.dart:376:9)
#3      VmService._routeRequest (package:vm_service/src/vm_service.dart:2376:27)
#4      VmService._processRequest (package:vm_service/src/vm_service.dart:2346:25)
#5      VmService._processMessageStr (package:vm_service/src/vm_service.dart:2313:9)
#6      VmService._processMessage (package:vm_service/src/vm_service.dart:2271:7)
#7      _RootZone.runUnaryGuarded (dart:async/zone.dart:1618:10)
#8      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:34…
[5:03:05 PM] [General] [Error] DevTools failed to launch Chrome, will launch default browser locally instead: Failed to launch browser: type 'Null' is not a subtype of type 'String'
#0      DevToolsServer.launchDevTools (package:dds/devtools_server.dart:507:11)
#1      MachineModeCommandHandler.registerLaunchDevToolsService.<anonymous closure> (package:dds/src/devtools/machine_mode_command_handler.dart:378:26)
#2      MachineModeCommandHandler.registerLaunchDevToolsService.<anonymous closure> (package:dds/src/devtools/machine_mode_command_handler.dart:376:9)
#3      VmService._routeRequest (package:vm_service/src/vm_service.dart:2376:27)
#4      VmService._processRequest (package:vm_service/src/vm_service.dart:2346:25)
#5      VmService._processMessageStr (package:vm_service/src/vm_service.dart:2313:9)
#6      VmService._processMessage (package:vm_service/src/vm_service.dart:2271:7)
#7      _RootZone.runUnaryGuarded (dart:async/zone.dart:1618:10)
#8      _BufferingStreamSubscription._sendData (dart:async/stream_impl…

The issue appears to be that we're not passing page and this code is now null-safe and assumes a page. It's not clear from the docs before that null was really intended to be a valid value, so we should fix VS Code to always provide a page ID.