cant Go to second navigation page from first navigation page after getting response data from server

Hi,

I want to go one navigation page to other navigation page after getting response from server.

My code is below,

private void LoginBtnCliked(object sender, EventArgs e)
    {
        API.AuthenticationCompleted += API_AuthenticationCompleted;

        //call service call from below method
        API.AuthenticateUser(txtEntryEmailAdd.Text, txtEntryPwd.Text);
    }

//calls after getting response from server
void API_AuthenticationCompleted(User user)
    {
        System.Diagnostics.Debug.WriteLine("In LoginPage :: API_AuthenticationCompleted() :: ");

        API.AuthenticationCompleted -= API_AuthenticationCompleted;

        // Do stuff here like change view it user is not null

        if (user != null)
        {

            System.Diagnostics.Debug.WriteLine("In LoginPage :: user not null 11111");

            //call after server response...
            var navigationPage = new ChoosePressPage();
            this.Navigation.PushAsync(navigationPage);
        }
    }     

I cant go to Choose Press page from Login page. Please help me

Thanks,