[MV3] Fix late initialization error on debugger detach (#1879)

Author: elliette

dwds/debug_extension_mv3/web/debug_session.dart

@@ -424,15 +424,15 @@ class _DebugSession {
   // What triggered the debug session (debugger panel, extension icon, etc.)
   final Trigger? trigger;
 
-  // The tab ID that contains the corresponding Dart DevTools.
-  late final int? devToolsTabId;
-
   // Socket client for communication with dwds extension backend.
   late final SocketClient _socketClient;
 
   // How often to send batched events.
   static const int _batchDelayMilliseconds = 1000;
 
+  // The tab ID that contains the corresponding Dart DevTools, if it exists.
+  int? devToolsTabId;
+
   // Collect events into batches to be send periodically to the server.
   final _batchController =
       BatchedStreamController<ExtensionEvent>(delay: _batchDelayMilliseconds);