undefined method WC_Product_Simple::get_visible_children()

When creating a new variable product, we get the following error (with WP_DEBUG active):
Fatal error: Call to undefined method WC_Product_Simple::get_visible_children() in /var/www/web347/htdocs/wp-content/plugins/giftable-for-woocommerce/admin/ScreenEditProduct.php on line 196
(The product gets created as a regular product).

Changing a regular product to a variable product also fails as long as this plugin is enabled.

Website runs on WordPress 4.9.9, using Divi 2.5.3 with a child theme.
Installed Plugins (not all are enabled)
ARI Fancy Lightbox (1.3.6)
All In One SEO Pack (2.10.1)
Antispam Bee (2.9.0)
Auto Thickbox (3.5)
B2B Market (1.0.2)
Better Search Replace (1.3.2)
Contact Form 7 (5.1.1)
Contact Form DB (2.9.13)
Duplicator (1.3.2)
Giftable for WooCommerce (1.0.2)
Google XML Sitemap (0.1.1)
Google XML Sitemaps (4.1.0)
InfiniteWP – Client (1.8.5)
LayerSlider WP (5.3.2)
Lightbox Plus Colorbox (2.7.2)
Novalnet payment plugin – WooCommerce (11.2.9)
Price based on User Role for WooCommerce (1.2.2)
Redirection (3.7.2)
SMTP Mailer (1.0.5)
Shopanpassungen (1.0)
Shortcodes by Angie Makes (3.45)
Simple CSS (1.1)
TM WooCommerce Compare & Wishlist (1.1.7)
User Role Editor (4.49)
WC – APG Free Shipping (2.2.1.2)
WC Hide Shipping Methods (1.3)
WP Colorful Tag Cloud (2.0.1)
WP HTML Mail (2.8.4.2)
WP Mail SMTP Plugin by Mail Bank (4.0.5)
WP PayPal (1.1.0)
WassUp Real Time Analytics (1.9.4.4)
Wish List for WooCommerce (1.5.2)
WooCommerce (3.5.3)
WooCommerce Advanced Free Shipping (1.1.3)
WooCommerce Germanized (2.2.7)
WooCommerce PDF Invoices & Packing Slips (2.2.5)
WooCommerce Quantity Increment (1.1.0)
WooCommerce Role Based Price (3.3.3)
WooCommerce Variation Swatches (1.0.4)
WooCommerce Variation Swatches (1.0.52)
WooCommerce Wishlist Plugin (1.9.9)
Woocommerce Minimum and Maximum Quantity (2.0.3)
Woocommerce Table Rate Shipping (2.0.3)
iThemes Security (7.2.0)