In the data model editor, the Design tab provides a graphical view of an existing data model and allows you to design a data model from scratch using the graphical designer. However, this functionality is still in evolution, and you may encounter the problem of not being able to open the data model correctly in the Design tab.
When it happens, the Design tab is displayed as "Loading..." and the graphical data model designer opens empty.
To resolve the problem, you can close the data model editor first and then delete the corresponding .er file of the data model under the directory <Project_Name>/MDM/datamodel in the Navigator view.
Alternatively, you can save the data model and reopen it.
If the Navigator view is not available in the Studio, select Window > Show View from the menu bar to open the [Show View] dialog box, and then enter Navigator in the search field and click OK to open the Navigator view.