It does not show my boostrap modal

I am working with FullCalendar, the problem I have is the following:

I have a button to add records, when I press it, it opens the modal, but if I close it and press the button again, I no longer leave the modal left as a line at the top.

Annex images, I hope you can help me.

repo: link

This is my code: Button:

<button type="button" class="btn btn-sm btn-warning btnAddUser" onclick="valida_envia()" <i class='glyphicon glyphicon-plus'></i> Agregar</button>

<script>
function valida_envia(){  
                    $('#newEvent').modal('show');
                }  
</script>

Modal: 


<form id="nuevoEvento" method="post">
    <div id="newEvent" class="modal" tabindex="-1" role="dialog">       
        <div class="modal-dialog" role="document">
            <div class="modal-content">
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
                    <h4 class="modal-title">Nuevo Registro</h4>
                </div>
                <div class="modal-body"> 
                    <div id="datos_ajax_event"></div>                        
                    <div class="panel panel-default">
                        <div class="panel-heading">
                            <h3 class="panel-title">Datos de Evento</h3>
                        </div>
                        <div class="panel-body"> 
                        </div>   
                    </div>
                    <div class="modal-footer" style="margin-top: -25px">
                    <button type="submit" class="btn btn-primary">Guardar</button>
                    <button type="button" class="btn btn-default" data-dismiss="modal" data-backdrop="false">Cerrar</button>                    
                </div>
            </div><!-- /.modal-dialog -->
        </div><!-- /.modal -->         
    </div>          
</form>