If you get this model error message, an action has been orphaned in your model.

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

Make one (probably the only one) of the expressions there an Action (1)

Paste the code below (2)

Execute it using F5 or the button (3)

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

Close the dialog and save you model as usual.