What's the cause of Synchronous XMLHttpRequest?

Question

I didn't know where this alert came from and what's the cause. I'm just trying to append TinyMCE in my panel-body based on the click of the user. It's fully working. But when I check the Console it says that.

Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience.

Here where I just extend the master page so I will not load the javascript again.

@section ('page-script')

<script>
    $(document).on("click", "#newDocuments", function ()
    {
        $.get('blankDocuments', function(data)
        {
            $("#documentBody").empty();
            $("#documentBody").append(data);
        });
    });
</script>

@endsection

This is where I append the TinyMCE in my panel-body and cause the alert in Console.

blankDocuments.blade.php

<div class = "form-group">
<h3>Input</h3>
<textarea class = "form-control" name = "content" id = "mytextarea" rows = "10"></textarea>
</div>

<script type = "text/javascript" src = "{{ URL::asset('js/jquery.js') }}"></script>
<script src = "{{ URL::to('src/js/tinymce/js/tinymce/tinymce.min.js') }}"></script>

<script>
tinymce.init
({
    selector: 'textarea',
    plugins: 
    [
        'advlist autolink link image lists charmap print preview hr anchor pagebreak spellchecker',
        'searchreplace wordcount visualblocks visualchars code fullscreen insertdatetime media nonbreaking',
        'save table contextmenu directionality emoticons template paste textcolor'
    ],
});
</script>