private function normalizeStylesheetsProperty()
    {
        if (empty($this->pluginOptions['stylesheets']))
            $this->pluginOptions['stylesheets'] = array();
        else if (is_array($this->pluginOptions['stylesheets']))
            $this->pluginOptions['stylesheets'] = array_filter($this->pluginOptions['stylesheets'], 'is_string');
        else if (is_string($this->pluginOptions['stylesheets']))
            $this->pluginOptions['stylesheets'] = array($this->pluginOptions['stylesheets']);
        else
            $this->pluginOptions['stylesheets'] = array();
    }