I am following the Trailhead tutorial on creating Lightning Components. It says that this requires the Salesforce org to have a domain. Well, as a matter of fact, we have one. And so do our developer sandboxes.
For example, the URL is https://companyname--sandboxname.cs88.my.salesforce.com/_ui/common/apex/debug/ApexCSIPage
However, when clicking the preview button in the Developer Console, I get an error message that Lightning Components require a My Domain.
What gives? How can I develop in a sandbox?