`if (empty($favicon_url)) { $blavatar = ''; } else { echo

if (empty($favicon_url)) {
                $blavatar = '';
            } else {
                echo '<style>#wpadminbar .quicklinks li .blavatar{width: 16px;height: 16px;padding-right: 5px;padding-top: 5px;}</style>';
                $favicon_url = preg_replace(array('/http:/i', '/https:/i'), '', $favicon_url);
                if ($favicon) {
                    $favicon_url = $favicon;
                }
                $blavatar = '<img src="' . $this->get_url_valid_shema(esc_url($favicon_url)) . '" alt="' . esc_attr__('Blavatar') . '" width="16" height="16" class="blavatar"/>';
            }

In wp-content/plugins/ultimate-branding/ultimate-branding-files/modules/favicons.php

Your currently leaving it as an undefined variable when the user doesn't input a favicon url. Please replace with the above code, thanks.