Problem with javascript form

Hi, I have a form in html and javascript that gives me the following error: Can not read property'value'of null and I do not know why I have attached the html and the js Html:

<form accept-charset="UTF-8" action="" method="post" onsubmit="enviaTelefonoNombre(); return false">
   <h2></h2>
   <div class="row">
      <input class="span3" name="nombre" placeholder="nombre" type="text" id="nombrelp" required="" class="form-control podo-input" style="font-size:12px;height: 33px; margin-bottom: 0px; margin-top: 0px; text-align: center; width: 100%; background: white; border-radius: 20px; border: 1px solid #706f6f; min-width: 142px;margin-bottom:5%;">
      <br>
   </div>
   <div class="row">
      <input class="span3" name="telefono" placeholder="teléfono" type="tel" id="telefonolp" pattern="^(6|7|9)\d{8}" class="form-control podo-input" required="" style="font-size:12px;height: 33px; margin-bottom: 0px; margin-top: 0px; text-align: center; width: 100%; background: white; border-radius: 20px; border: 1px solid #706f6f; min-width: 142px;">
      <br>
   </div>
   <input type="hidden" id="hiddenFormField" name="custId" value="nulo" style="float:left;padding: 4px 32px 4px 32px; margin: 0px; color: white; margin-right: 10px; background-image: linear-gradient(to right,#ff00b9, #ff0800); border-radius: 20px; border: 0; width: 100%; max-width: 150px; min-height: 33px;">
   <br>  <button class="boton" type="submit" style="float:left;padding: 4px 32px 4px 32px; margin: 0px; color: white; margin-right: 10px; background-image: linear-gradient(to right,#ff00b9, #ff0800); border-radius: 20px; border: 0; width: 100%; min-height: 33px;">Te llamamos</button>
   <label><br><input name="acepto" type="checkbox" value="checkbox" required="" style="left:-10px;"> Acepto la<a href="/politica-de-privacidad/" name="submit" target="&quot;_blank" type="submit" value="Enviar"> política de privacidad</a></label>
</form>

JS:

<script type="text/javascript"> 
function enviaTelefonoNombre(){

    //Recuperación datos formulario

    var phone=document.getElementById("telefonolp").value;
    var name=document.getElementById("nombrelp").value;
    var hidden=document.getElementById("hiddenFormField").value;
    //var email=document.getElementById("email").value;

    endpoint(phone,name,hidden,function(response){
        console.log(response);
        if(response===201) {
            dataLayer.push({event:'calcula'});
            alert("¡Gracias! Hemos recibido tu petición");
            window.location.reload(false);
        } else{
            alert("Lo sentimos, no hemos podido procesar tu solicitud");
            window.location.reload(false);
        }
    });

}
function endpoint(phone,name,hidden,callback4){

    // Envío datos a callcenter

    var data = "name=Afiliacion_web"+name+"&phone="+phone+"&source="+"&[email protected]"+hidden;

    console.log(data);

    var xhr = new XMLHttpRequest();

    xhr.open("POST", "https://backoffice.mipodo.com/api/contactforms/");
    xhr.setRequestHeader("content-type", "application/x-www-form-urlencoded; charset=UTF-8");
    xhr.setRequestHeader("accept", "*/*");


    xhr.send(data);

    xhr.onreadystatechange = function()
    {

            if (xhr.readyState == 4)
                if(xhr.status == 200){
                    var response = xhr.responseText;
                    responseJsonOferta = JSON.parse(response);
                    //console.log('dentro: '+responseJsonOferta);

                    return callback4(responseJsonOferta);

                }else {
                    return callback4(JSON.parse(xhr.status));
                }




    }
}
</script>