Topic
You can resolve this by following the instructions and documentation linked below.
Instructions
As a workaround: from inside Lightning, click on your user avatar and select "Switch to Salesforce Classic." Once in Classic, navigate to Nintex Apps/Skuid.
There is a product fix available as of version 17.11.17.
You will need to test the latest release of Nintex Apps for Salesforce thoroughly in a representative sandbox to confirm that your solutions work as expected. Note: there is no way to downgrade to an older managed package version once an upgrade is installed, so please ensure you are only logged into the org you want to upgrade before proceeding.
Be sure to follow these best practices when upgrading:
https://docs.skuid.com/latest/appdev/en/skuid/upgrade/#upgrading-skuid-sfx