Drop an AnchorPane, and Save.
Check the fxml file: there is no "fx:controller" property, as expected.
In SB, set the controllerClass to "controller", and Save
Check the fxml file: there is the "fx:controller" property, as expected.
Reset the controllerClass property (using cog>reset to default)
Check the fxml file: fx:controller="", when it should not be there => Bug

Same issue if emptying the controllerClass from its text field: the controllerClass property should not appear in the fxml file.