JForm::getInstance impossible de charger le fichier

1. Vérifiez d'abord votre forme de fichier xml

Avant la fin de l'path: components\com_<name_of_component>\models\forms\<formname>.xml

Back-end path: administrator\components\com_<name_of_component>\models\forms\<formname>.xml

Exemple (Joomla 3.4.3): administrator\components\com_users\models\forms\group.xml

<?xml version="1.0" encoding="utf-8"?>
<form>
    <fieldset>
        <field name="id" type="hidden"
            default="0"
            required="true"
            readonly="true"
        />

        <field name="title" type="text"
            required="true"
            description="COM_USERS_GROUP_FIELD_TITLE_DESC"
            label="COM_USERS_GROUP_FIELD_TITLE_LABEL"
            size="40"
        />

        <field name="parent_id" type="groupparent"
            description="COM_USERS_GROUP_FIELD_PARENT_DESC"
            label="COM_USERS_GROUP_FIELD_PARENT_LABEL"
            required="true"
        />

        <field name="actions" type="hidden"
            multiple="true"
        />

        <field name="lft" type="hidden"
            filter="unset"
        />
        <field name="rgt" type="hidden"
            filter="unset"
        />
    </fieldset>
</form>

2. Vérifiez votre modèle de classe

La méthode getForm() est responsable du chargement du formulaire (le formulaire xml fichier mentionné ci-dessus).

Avant la fin de l'path: components\com_<name_of_component>\models\<modelname>.php

Back-end path: administrator\components\com_<name_of_component>\models\<modelname>.php

Exemple (Joomla 3.4.3): administrator\components\com_users\models\group.php

/**
 * Method to get the record form.
 *
 * @param   array    $data      An optional array of data for the form to interogate.
 * @param   boolean  $loadData  True if the form is to load its own data (default case), false if not.
 *
 * @return  JForm   A JForm object on success, false on failure
 *
 * @since   1.6
 */
public function getForm($data = array(), $loadData = true)
{
    //Get the form.
    $form = $this->loadForm(
        'com_users.group', 
        'group', 
        array(
            'control' => 'jform', 
            'load_data' => $loadData)
        );

    if (empty($form))
    {
        return false;
    }

    return $form;
}