chore: Fix compilation error for web.

Author: cloudwebrtc

lib/src/transports/websocket_dart_impl.dart

@@ -13,7 +13,7 @@ typedef OnOpenCallback = void Function();
 class WebSocketImpl {
   WebSocketImpl(this._url);
 
-  final String? _url;
+  final String _url;
   WebSocket? _socket;
   OnOpenCallback? onOpen;
   OnMessageCallback? onMessage;
@@ -26,9 +26,9 @@ class WebSocketImpl {
     try {
       if (webSocketSettings.allowBadCertificate) {
         /// Allow self-signed certificate, for test only.
-        _socket = await _connectForBadCertificate(_url!, webSocketSettings);
+        _socket = await _connectForBadCertificate(_url, webSocketSettings);
       } else {
-        _socket = await WebSocket.connect(_url!,
+        _socket = await WebSocket.connect(_url,
             protocols: protocols, headers: webSocketSettings.extraHeaders);
       }
 

lib/src/transports/websocket_interface.dart

@@ -66,6 +66,11 @@ class WebSocketInterface implements Socket {
   @override
   void connect() async {
     logger.debug('connect()');
+
+    if (_url == null) {
+      throw AssertionError('Invalid argument: _url');
+    }
+
     if (isConnected()) {
       logger.debug('WebSocket $_url is already connected');
       return;
@@ -78,7 +83,7 @@ class WebSocketInterface implements Socket {
     }
     logger.debug('connecting to WebSocket $_url');
     try {
-      _ws = WebSocketImpl(_url);
+      _ws = WebSocketImpl(_url!);
 
       _ws!.onOpen = () {
         _closed = false;