Problem when starting modal for the second time

I have a button that calls a function, and opens the modal at the same time. The first time I do it, it does it well but the second time I run the button, I see this error in the console:

  

Type error: can not read property parentnode of null.

I leave the code.

This is the button in html, I open the modal.

<button type="button" id ="btnConvModal" class="btn btn-primary btn-xs " data-toggle="modal" data-target="#modalConvenios"><strong>Ver Convenios</strong></button>

I call the function

 $("#btnConvModal").on( "click", function() {
        busConveniosModal();
    });

This is the function:

function busConveniosModal(){ //Funcion que me lista una tabla con los convenios en un formulaio modal de administrador.
        $('#modalConvenios').on('shown.bs.modal', function (e) {
            tblConvenios = $('#tblConvenios').DataTable({ //Voy a necesitar la variable tabla
            "responsive": {
            "details": false,
            },

            autoWidth: false,
            "processing": true,
            "pagingType": "simple", //Hace que se muestren todos los elementos de la paginación               
            "destroy": "true", //Esto lo hago para que no me de error el datatable cuando lo vuelvo a inicializar una vez cargado.
            "language": {
                "url": "https://cdn.datatables.net/plug-ins/1.10.11/i18n/Spanish.json"
            },

            "ajax": {
                "url": "php/AgrApp/cnv_emp.php", //web a la que llamo y hace el trabajo.
                "type": "POST", //Metodo usado por la funcion para pasar variable
                "dataSrc": "",
            },


            "columns": [
                {"defaultContent":" <button type='button' id=\"idConvenioAdmin\"  class='selectConv  btn btn-primary btn-xs '><span class='glyphicon glyphicon-plus'></span></button>","width": "3%"},
                { "data": "id"  },
                { "data": "name" },
                ]
            });
     });

}

By making an alert ("test"); When I opened the modal, I realized that the second time I opened it, I ran the alert twice. I've put this line

$('#modalConvenios').off('shown.bs.modal'); 

and the problem has been solved, what I do not understand is the reason of the problem, it is how they were adding or storing $('#modalConvenios').on('shown.bs.modal', function (e) { every time I call it.