Handle race condition when HttpListener is stopped or closed (#63394)

* Throw a HttpListenerException if the session is null

* Add similar check to BeginGetContext

* Address PR feedback

* Remove trailing spaces

src/libraries/System.Net.HttpListener/src/System/Net/Windows/HttpListener.Windows.cs

@@ -508,7 +508,14 @@ public HttpListenerContext GetContext()
                 uint size = 4096;
                 ulong requestId = 0;
                 memoryBlob = new SyncRequestContext((int)size);
-                HttpListenerSession session = _currentSession!;
+                HttpListenerSession? session = _currentSession;
+
+                // Because there is no synchronization, the listener can be stopped or closed while the method is executing,
+                // resulting in a null session
+                if (session == null)
+                {
+                    throw new HttpListenerException((int)Interop.HttpApi.ERROR_INVALID_PARAMETER);
+                }
 
                 while (true)
                 {
@@ -617,10 +624,20 @@ public IAsyncResult BeginGetContext(AsyncCallback? callback, object? state)
                 {
                     throw new InvalidOperationException(SR.Format(SR.net_listener_mustcall, "Start()"));
                 }
+
+                HttpListenerSession? session = _currentSession;
+
+                // Because there is no synchronization, the listener can be stopped or closed while the method is executing,
+                // resulting in a null session
+                if (session == null)
+                {
+                    throw new HttpListenerException((int)Interop.HttpApi.ERROR_INVALID_PARAMETER);
+                }
+
                 // prepare the ListenerAsyncResult object (this will have it's own
                 // event that the user can wait on for IO completion - which means we
                 // need to signal it when IO completes)
-                asyncResult = new ListenerAsyncResult(_currentSession!, state, callback);
+                asyncResult = new ListenerAsyncResult(session, state, callback);
                 uint statusCode = asyncResult.QueueBeginGetContext();
                 if (statusCode != Interop.HttpApi.ERROR_SUCCESS &&
                     statusCode != Interop.HttpApi.ERROR_IO_PENDING)