Wordpress widget admin form updates values too late

I have a problem in a wirdpress widget:
When i save the (edited) widget data in wp backend, the widget fields (of only this widget) are empty on the automatically loaded (same) page. If i refreh the browser then, the fields contain the new values.
In the frontent, the widget does never display anything ($instance['title'] and $instance['category'] are empty.

<?php
class ranger_calendar_widget_list extends WP_Widget {
    protected $calendar;
    protected $months;
    public function __construct() {
        parent::__construct('',
        __('Calendar', 'ranger-calendar'),
        array(
                'classname' => 'ranger_calendar_widget_list',
                'description' => __('Displays the calendar as a list.', 'ranger-calendar'),
            ));
        $this -> calendar = new ranger_calendar();
        $this -> months = array(
            1 => __('January'),
            2 => __('February'),
            3 => __('March'),
            4 => __('April'),
            5 => __('May'),
            6 => __('June'),
            7 => __('July'),
            8 => __('August'),
            9 => __('September'),
            10 => __('October'),
            11 => __('November'),
            12 => __('Decenber')
        );
    }
    /**
     * Displays the widget's content
     */
    function widget($args, $instance) {
        $title = apply_filters('widget_title', $instance['title']);
        $category = $instance['category'];
        echo $args['before_widget'];
        echo $args['before_title'] . $instance['title'] . $args['after_title'];
        print_r($instance);
        if (!$category) {
            $category = 0;
        }
        $dates = $this -> calendar -> get_future_dates();
        $dates_final = array();
        if ($category != 0) {
            foreach ($dates as $date) {
                if ($date['category'] == $category) {
                    $dates_final[] = $date;
                }
            }
        } else {
            $dates_final = $dates;
        }
        echo $this -> generate_html($dates);
        echo $args['after_widget'];
    }
    public function generate_html($dates) {
        $html = "<ol class="date-list">";
        foreach ($dates as $date) {
            $id = $date['id'];
            $beginning = $date['beginning'];
            $datetime = date_create($beginning);
            $title = $date['title'];
            $html .= "<li>";
            $html .= "<time datetime="" . $beginning . "">";
            $html .= "<span class="month">" . $this->months[intval(date_format($datetime, "m"))] . "</span>";
            $html .= "<span class="day">" . date_format($datetime, "d") . "</span>";
            $html .= "</time>";
            $html .= $title;
            $html .= "</li>";
        }
        $html .= "</ol>";
        return $html;
    }
    public function form($instance) {
        // Title field
        if (isset($instance['title'])) {
            $title = $instance['title'];
        } else {
            $title = __('Dates', 'ranger-calendar');
        }
        echo "<p>";
        echo "<label for='" . $this -> get_field_id('title');
        echo "'>" . __('Title', 'ranger-calendar') . "</label> ";
        echo "<input type='text' id='" . $this -> get_field_id('title');
        echo "' name='" . $this -> get_field_id('title');
        echo "' value='" . esc_attr($title) . "' />";
        echo "</p>";
        // Category field
        if (isset($instance['category'])) {
            $category = $instance['category'];
        } else {
            $category = 0;
        }
        $categories = $this -> calendar -> get_categories();
        echo "<p>";
        echo "<label for='" . $this -> get_field_id('category');
        echo "'>" . __('Category', 'ranger-calendar') . "</label> ";
        echo "<select name='" . $this -> get_field_id('category');
        echo "' id='" . $this -> get_field_id('category');
        echo "'>";
        echo "<option value='0'";
        if($instance['category'] == '0') {
            echo " selected";
        }
        echo ">" . __('None', 'ranger-calendar') . "</option>";
        foreach($categories as $category) {
            echo "<option value='" . $category['id'] . "'";
            if($category['id'] == $instance['category']) {
                echo " selected";
            }
            echo ">" . $category['name'] . "</option>";
        }
        echo "</select>";
        echo "</p>";
    }
    public function update($new_instance, $old_instance) {
        $instance = array();
        $instance['title'] = (!empty($new_instance['title'])) ? strip_tags($new_instance['title']) : '';
        $instance['category'] = (!empty($new_instance['category'])) ? strip_tags($new_instance['category']) : '';
        return $instance;
    }
}
add_action('widgets_init', 'register_ranger_calendar_widget_list');
function register_ranger_calendar_widget_list() {
    register_widget('ranger_calendar_widget_list');
}