The IDE maintenance on Thursday introduced a bug where users are unable to create a new model using the Device Settings dialog box. If you try to do that, the device will disappear from the sidebar and no new model will be created. Refreshing the page will restore things to the previous state.
This issue will be fixed in our next maintenance window, which should be on Tuesday.
In the meantime, please use the Create New Model button, and assign the device using the checkbox list or just create the model and then assign the device manually (assigning a device to existing models using the Device Settings dialog box works fine.)
Sorry for the inconvenience! I’ll update this post when the issue is resolved.