Erro ao pegar Collection de um servlet

Tenho essa classe que pega uma coleção de um servlet mas está ocorrendo um erro na linha 20 e não consigo descobrir o que pode ser.
Essa é a mensagem de erro: Exception in thread “main” java.io.IOException: Server returned HTTP response code: 500 for URL: http://localhost:8080/WebApplication3/TesteServlet at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1241)

public class TesteHttp {
    
    public void teste() throws IOException, ClassNotFoundException{
       
        String usuario = "admin";

        java.net.URL netUrl = new java.net.URL("http://localhost:8080/WebApplication3/TesteServlet");
        HttpURLConnection connection = (HttpURLConnection)netUrl.openConnection();
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Length", "" + Integer.toString(usuario.getBytes().length));
        connection.setUseCaches(false);
        connection.setDoInput(true);
        connection.setDoOutput(true);   
        
        DataOutputStream out = new DataOutputStream(connection.getOutputStream());
        out.writeBytes(usuario);
        out.flush();
        out.close();
        
        ObjectInputStream ois = new ObjectInputStream(connection.getInputStream()); // algo errado aqui
        Collection <String> c = (Collection)ois.readObject();
        Iterator it = c.iterator();
        while (it.hasNext()){
            System.out.println((String)it.next());
        }
}

Código do servlet:

public class TesteServlet extends HttpServlet {
   

    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
            
        DataInputStream in = new DataInputStream(request.getInputStream()); 
        String usuario = in.readLine();        
         
       if (usuario.equals("admin")){
            Collection <String> c = null;
            c.add("teste");
        
            ObjectOutputStream out = new ObjectOutputStream(response.getOutputStream());
            out.writeObject(c);
            out.flush();
            out.close();
        }
}

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