Google map - Not loading when refresh page

Hi people.

I wanted to try to google map API in an application that :

• Create an User (Email, adress, city, postal code),
• Define the latitude & longitude from the adress,
• Put a mark on google map with this adress.

Actually, it's working well, you can even try on my site : mapuser.leroy-thomas.fr

But I found a problem.

Sometimes, when you reload the page, the map don't want to appear, and in the console, I got an error :

Uncaught 
Qb {message: "initMap is not a function", name: "InvalidValueError", stack: "Error↵    at new Qb (https://maps.googleapis.com/m…KHdBCcpmkMnyrgIbpupNBQL5c&callback=initMap:150:59"}
message:"initMap is not a function"
name:"InvalidValueError"

I don't know why it's doing that, because it's a bit random, sometimes I can reload the page 10 times without any erros, and after the page don't want to reload.

If you want my JS code :

<script type="text/javascript">
        var map;

        function initMap() {
            map = new google.maps.Map(document.getElementById('map'), {
                center: {lat: 48.764651, lng: 0.628715},
                zoom: 8
            });
        }

        function addMarker(location) {
            marker = new google.maps.Marker({
                position: location,
                map: map
            });
        }

        function marker(){
            City = new google.maps.LatLng(48.764651, 0.628715);
            addMarker(City);
        }

        $.ajax({
            url: "/mapuser/getUser",
            type: "GET",
            dataType: "json",
            success: function( response ){

                $.each( response, function( index, value ){
                    City = new google.maps.LatLng(value.latitude, value.longitude);
                    addMarker(City);
                });
            }
        });
    </script>

Well, you can try on the website, and if you have an idea about why it's doing that, and what to do for solve this problem, I thank you in advance.