Notice: Undefined index: dhl_tracking_url_language in /home/cocukaka/public_html

Hello,

I have DHL plugin in my store and recently may or may not be related to a hosting transfer I’m getting the following error.
“Notice: Undefined index: dhl_tracking_url_language in /home/cocukaka/public_html/wp-content/plugins/dhl-for-woocommerce/includes/class-pr-dhl-wc-order-paket.php on line 559”

At first, when the servers exchanged there was no problem with the plugin and then suddenly I started to see this error on woocommerce orders page then I press ‘download label’ inside the order itself , it downloads an HTML file named “download.html”. Content of the HTML is the source code of website’s main page.

I know it sounds someone did something but I don’t have any idea how and who.

Thanks for the answers in advanced.