Avoid InvalidCastException and guard for null

Author: nvborisenko

dotnet/src/webdriver/BiDi/WebDriver.Extensions.cs

@@ -18,6 +18,7 @@
 // </copyright>
 
 using OpenQA.Selenium.BiDi.Modules.BrowsingContext;
+using System;
 using System.Threading.Tasks;
 
 #nullable enable
@@ -28,11 +29,18 @@ public static class WebDriverExtensions
 {
     public static async Task<BiDi> AsBiDiAsync(this IWebDriver webDriver)
     {
-        var webSocketUrl = ((IHasCapabilities)webDriver).Capabilities.GetCapability("webSocketUrl");
+        if (webDriver is null) throw new ArgumentNullException(nameof(webDriver));
+
+        string? webSocketUrl = null;
+
+        if (webDriver is IHasCapabilities hasCapabilities)
+        {
+            webSocketUrl = hasCapabilities.Capabilities.GetCapability("webSocketUrl")?.ToString();
+        }
 
         if (webSocketUrl is null) throw new BiDiException("The driver is not compatible with bidirectional protocol or \"webSocketUrl\" not enabled in driver options.");
 
-        var bidi = await BiDi.ConnectAsync(webSocketUrl.ToString()!).ConfigureAwait(false);
+        var bidi = await BiDi.ConnectAsync(webSocketUrl).ConfigureAwait(false);
 
         return bidi;
     }