Do you really want to use id as the editor data field? The id refers to the component's programatic variable name, right?
Even so, that said; the editorDataFiel property refers to a property on the editor, not on properties within the editor. I did not think it would drill down to use 'selectedItem.id' ; but you can use 'selectedItem.
You could also extend the ComboBox to have a 'value' type of property that holds 'selectedItem.id" from your ComboBox's dataProvider. Except rename the 'id' property.