Open attached text.fxml
Select Button 1 of the root container
Now make a Marquee selection, originating from the top left corner of the root AP and dragging in a SE direction - bug - the following exception occurs:

SEVERE: An exception was thrown:
java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController.getFXOMInstance(CssPanelController.java:1528)
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController.fillSelectionContent(CssPanelController.java:687)
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController.access$2100(CssPanelController.java:108)
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController$8.run(CssPanelController.java:651)
at com.sun.javafx.application.PlatformImpl$6$1.run(PlatformImpl.java:301)
at com.sun.javafx.application.PlatformImpl$6$1.run(PlatformImpl.java:298)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl$6.run(PlatformImpl.java:298)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)