How to start another window using eel python?

I am creating a GUI application using eel library in python. index.html contains login form and if login is successful I want to open retrieve.html.

This is my python code and please consider login as successful.

@eel.expose
def retrieve():
    eel.start('retrieve.html', size=(1000, 700))


eel.start('index.html', size=(1000, 700))

This is my javascript code

function login_func()
{
    var username = document.getElementById("username").value
    var pword = document.getElementById("pword").value
    eel.login(username, pword)(set_result)
}

function set_result(result)
{
    if(result == "Failed")
    {
        window.alert("Please insert correct username and password")
    }
    else
    {
        window.close()
        eel.retrieve()
    }
}

Everything works fine but I am getting an error message and that is

OSError: [WinError 10048] Only one usage of each socket address (protocol/network address/port) is normally permitted: ('localhost', 8000)

How can I avoid this?