Hi,
I ran into the same issue. The fix was to put a type check on the $product_obj variable on line 231 of the class-woo-product-stock-alert-admin.php file. So, change this:
if( $product_obj->is_type('variable') ) {
…to this:
if( (gettype($product_obj) == "object") && $product_obj->is_type('variable') ) {
