Custom App.Config configSections

I'm trying to add customs values, outside of appSettings, because I need to map these values and take them as part of an object ...

In this same forum but in English, I read how to do it and try to replicate it without success .. this is my App.Config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <!-- Configuracion de Email -->
    <add key="UserEmail" value="[email protected]" />
    <add key="PasswordEmail" value="123456" />
    <!-- Configuracion de estructura de Email -->
    <add key="SubjetEmail" value="Evento [{0}]" />
    <add key="BodyEmail" value="&lt;font size=3&gt;--- Se ha registrado un evento {0} ---&lt;p&gt;&lt;/p&gt;&lt;font color=blue&gt;&lt;u&gt;Información detallada&lt;/u&gt;&lt;/font&gt;&lt;p&gt;&lt;/p&gt;&lt;b&gt;{1}&lt;/b&gt;" />
  </appSettings>
  <!-- Cadenas de Conexion a DB -->
  <connectionStrings>
    <add name="mibasededatos" connectionString="Data Source=x.x.x.x;Initial Catalog=mibasededatos;user id=miusuario;password=mipassword" providerName="System.Data.SqlClient" />
  </connectionStrings>
  <configSections>
    <section name="urlAddresses" type="namespace.UrlRetrieverSection"/>
  </configSections >
  <!-- URL (Agregar o quitar) -->
  <urlAddresses>
    <add name="Google" url="http://www.google.com" />
  </urlAddresses>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
  </startup>
</configuration>

And the UrlRetrieverSection class

public class UrlRetrieverSection : ConfigurationSection
    {
        [ConfigurationProperty("", IsDefaultCollection = true, IsRequired = true)]
        public UrlCollection UrlAddresses
        {
            get
            {
                return (UrlCollection)this[""];
            }
            set
            {
                this[""] = value;
            }
        }
    }


    public class UrlCollection : ConfigurationElementCollection
    {
        protected override ConfigurationElement CreateNewElement()
        {
            return new UrlElement();
        }
        protected override object GetElementKey(ConfigurationElement element)
        {
            return ((UrlElement)element).Name;
        }
    }

    public class UrlElement : ConfigurationElement
    {
        [ConfigurationProperty("name", IsRequired = true, IsKey = true)]
        public string Name
        {
            get
            {
                return (string)this["name"];
            }
            set
            {
                this["name"] = value;
            }
        }

        [ConfigurationProperty("url", IsRequired = true)]
        public string Url
        {
            get
            {
                return (string)this["url"];
            }
            set
            {
                this["url"] = value;
            }
        }
    }

Then read it as follows

UrlRetrieverSection UrlAddresses = (UrlRetrieverSection)ConfigurationManager.GetSection("urlAddresses");

And here comes the problem, apparently the structure of the App.Config is corrupted, because the error the error is as follows.

Configuration system initialization failed

================================================================================================= ===============

In the end I was able to correct it. The problem is that

<configSections>
    <section name="urlAddresses" type="namespace.UrlRetrieverSection"/>
  </configSections >

It must be declared at the start of the App.Config, just after configuration and before appSettings.

Greetings.