The «WebKul Magento Canada Post Shipping extension» breaks my checkout

Context: How to diagnose a «Error log record number» failure?

As have unzipped and published the stack tracke for you, and, as you can see, the issue is caused by the WebKul Magento Canada Post Shipping extension.

a:4:{i:0;s:181:"Warning: Invalid argument supplied for foreach() in pp/code/Webkul/MpCanadapostShipping/Observer/SalesOrderPlaceAfterObserver.php on line 80";i:1;s:12508:"#0 pp/code/Webkul/MpCanadapostShipping/Observer/SalesOrderPlaceAfterObserver.php(80): Magento\Framework\App\ErrorHandler->handler(2, 'Invalid argumen...', '/chroot/home/yo...', 80, Array)
#1 vendor/magento/framework/Event/Invoker/InvokerDefault.php(73): Webkul\MpCanadapostShipping\Observer\SalesOrderPlaceAfterObserver->execute(Object(Magento\Framework\Event\Observer))
#2 vendor/magento/framework/Event/Invoker/InvokerDefault.php(61): Magento\Framework\Event\Invoker\InvokerDefault->_callObserverMethod(Object(Webkul\MpCanadapostShipping\Observer\SalesOrderPlaceAfterObserver), Object(Magento\Framework\Event\Observer))
#3 vendor/magento/framework/Event/Manager.php(66): Magento\Framework\Event\Invoker\InvokerDefault->dispatch(Array, Object(Magento\Framework\Event\Observer))
#4 app/code/Webkul/Marketplace/Observer/SalesOrderSuccessObserver.php(540): Magento\Framework\Event\Manager->dispatch('mp_order_save_a...', Array)
#5 app/code/Webkul/Marketplace/Observer/SalesOrderSuccessObserver.php(164): Webkul\Marketplace\Observer\SalesOrderSuccessObserver->productSalesCalculation(Object(Magento\Sales\Model\Order\Interceptor))