If you get this model error message, an action has been orphaned in your model. For example, a Class has been deleted, but, for some reason, the Class action belonging to it was left behind.

To remove actions without a name, open the OCL debugger for the model itself by right-clicking on a diagram background and selecting Extras -> Open model debugger...

1. Make one (probably the only one) of the expressions there an Action (1)
2. Paste the code below (2)
3. Execute it using F5 or the button (3)

AbstractAction.allInstances->select(aa|aa.Name.isNullOrEmpty).delete

Close the dialog and save your model as usual.