Call to a member function prepare () on a non-object in subrarchivo.phtml on line 15

I have the following error when I upload an excel file to the server:

  

Call to a member function prepare () on a non-object in ...

I show you my file code.

subrile.phtml

<div class="modal-body">

    <?php
    $form = $this->form;

    $form->prepare();

    $form = $this->form;
    $form->setAttribute('action', $this->url('tributaria/default', array('controller' => 'index', 'action' => 'subirarchivo')));
    $form->prepare();
    echo $this->form()->openTag($form);
    ?>
    <div class="row">
        <span class="col-sm-3 control-label"><?php echo $this->formLabel($form->get('file')); ?></span>
        <div class="col-sm-9"><?php echo $this->formFile($form->get('file')); ?></div>
    </div>


    <div class="text-center" style="background-color: #f8efc0; padding: 8px">
        <b>ATENCI&Oacute;N:</b><br> La versi&oacute;n del archivo EXCEL debe ser m&iacute;nimo MICROSOFT EXCEL 2007
    </div>
    <div class="text-center" style="background-color: #f8efc0; padding: 8px">
        Formato de columnas del archivo Excel<br> <b>[ITEM]  [CARNET_IDENTIDAD]  [NOMBRE_FUNCIONARIO]   [DIAS_NO_TRABAJADOS] [DIAS_HABILES] [DIAS_TRABAJADOS]</b>
    </div>

    <a href="/data/formato.xlsx" >Descargar Plantilla</a>

            <div class="modal-footer" style="background-color: #f0f0f0">

                  <?php echo $this->formElement($form->get('Registrar')) ?>
                <button type="button" class="btn default btn-xs" data-dismiss="modal"><?php echo $this->idTributaria == '0' ? 'Cancel' : 'Cerrar' ?></button>
            </div>

   <?php echo $this->form()->closeTag($form); ?>                                                
 </div>

javascript fragment

<script type="text/javascript">
            jQuery(document).ready(function () {

                handleFormBases();

            });

            function handleFormBases() 
            {
                var form1 = $('#frmTributaria');
                var error1 = $('.alert-danger', form1);
                var success1 = $('.alert-success', form1);

                form1.validate({
                    errorElement: 'span', //default input error message container
                    errorClass: 'help-block', // default input error message class
                    focusInvalid: false, // do not focus the last invalid input
                    ignore: "",
                    rules: {
                        file: {
                            required: true,
                            extension: "xls|xlsx",
                        },
                    },
                    messages: {// custom messages for radio buttons and checkboxes
                        file: {
                            required: "Debe cargar un archivo valido de excel",
                            extension: "Debe cargar un archivo valido de excel",
                        },
                    },
                    invalidHandler: function (event, validator) { //display error alert on form submit              
                        success1.hide();
                        error1.show();
                        App.scrollTo(error1, -200);
                    },
                    highlight: function (element) { // hightlight error inputs
                        $(element)
                                .closest('.form-group').addClass('has-error'); // set error class to the control group
                    },
                    unhighlight: function (element) { // revert the change done by hightlight
                        $(element)
                                .closest('.form-group').removeClass('has-error'); // set error class to the control group
                    },
                    success: function (label) {
                        label
                                .closest('.form-group').removeClass('has-error'); // set success class to the control group
                    },
                    submitHandler: function (form) {
                       //alert("aaaaa");
                        ajaxEnviar(form, '/tributaria/index/index', '#result');
                    }
                });

            }
  function ajaxEnviar(form, url, div) 
    {          
                    $('#modal_subirarchivo').modal('hide');


            var formData = new FormData(form);

                    $.ajax({
                        type: $(form).attr('method'),

                        url: $(form).attr('action'), 
                        cache: false,
                        contentType: false,
                        processData: false,
                        data: formData,
                        mimeType: "multipart/form-data",
                        dataType: "json",
                        success: function (data) 
                        {
                            if (data.respuesta) 
                            {

                                jQuery(div).load(url);

                                toastr.success(data.mensaje, 'SUCCESS');
                            } else {

                                toastr.error(data.mensaje, 'ERROR');

                            }

                        },
                        error: function (jqXHR, status, error) {
                            alert(error);
                            toastr.error('OCURRRIO UN ERROR AL EJECUTAR LA PAGINAss', 'ERROR');
                         },
                });

 }

I was doing tests and it does not enter the success of ajax, it goes straight to the error.