Ember 3.26+: TypeError: this.overlayClassNames?.join is not a function

[Thomasevano]

With Ember 3.26 and ember-modal-dialog i get this error trying to open the dialog

Uncaught (in promise) TypeError: this.overlayClassNames?.join is not a function at BasicDialog.get overlayClassNamesString (basic-dialog.js:59:1) at index.js:1936:1 at untrack (validator.js:836:1) at ComputedProperty.get (index.js:1935:1) at BasicDialog.getter [as overlayClassNamesString] (index.js:1025:1) at getPossibleMandatoryProxyValue (index.js:1326:1) at _getProp (index.js:1391:1) at reference.js:213:1 at reference.js:161:1 at track (validator.js:820:1)