Drop a Chart (as root element).
Take menubar > View > Show Sample Data
Delete the Chart

You get:

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.fxom.sampledata.SampleDataGenerator.assignSampleData(SampleDataGenerator.java:59)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.refreshSceneGraph(FXOMDocument.java:269)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.endUpdate(FXOMDocument.java:254)
at com.oracle.javafx.scenebuilder.kit.editor.job.v2.CompositeJob.execute(CompositeJob.java:101)
at com.oracle.javafx.scenebuilder.kit.editor.job.DeleteSelectionJob.execute(DeleteSelectionJob.java:62)
at com.oracle.javafx.scenebuilder.kit.editor.JobManager.executeJob(JobManager.java:181)
at com.oracle.javafx.scenebuilder.kit.editor.JobManager.push(JobManager.java:70)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.performEditAction(EditorController.java:839)
at com.oracle.javafx.scenebuilder.app.DocumentWindowController.performDelete(DocumentWindowController.java:1350)
at com.oracle.javafx.scenebuilder.app.DocumentWindowController.performEditAction(DocumentWindowController.java:807)
at com.oracle.javafx.scenebuilder.app.DocumentWindowController$1.handle(DocumentWindowController.java:308)
at com.oracle.javafx.scenebuilder.app.DocumentWindowController$1.handle(DocumentWindowController.java:247)
at com.sun.javafx.event.CompositeEventHandler$NormalEventFilterRecord.handleCapturingEvent(CompositeEventHandler.java:282)
at com.sun.javafx.event.CompositeEventHandler.dispatchCapturingEvent(CompositeEventHandler.java:98)
at com.sun.javafx.event.EventHandlerManager.dispatchCapturingEvent(EventHandlerManager.java:223)
at com.sun.javafx.event.EventHandlerManager.dispatchCapturingEvent(EventHandlerManager.java:180)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchCapturingEvent(CompositeEventDispatcher.java:43)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:52)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.scene.Scene$KeyHandler.process(Scene.java:3949)
at javafx.scene.Scene$KeyHandler.access$2100(Scene.java:3896)
at javafx.scene.Scene.impl_processKeyEvent(Scene.java:2036)
at javafx.scene.Scene$ScenePeerListener.keyEvent(Scene.java:2493)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:170)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:123)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleKeyEvent(GlassViewEventHandler.java:197)
at com.sun.glass.ui.View.handleKeyEvent(View.java:517)
at com.sun.glass.ui.View.notifyKey(View.java:927)