Drop a ToggleButton.
Edit its toggleGroup property.
The following assertion is raised:

java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController.editorSelectionDidChange(InspectorPanelController.java:488)
at com.oracle.javafx.scenebuilder.kit.editor.panel.util.AbstractPanelController$6.changed(AbstractPanelController.java:250)
at com.oracle.javafx.scenebuilder.kit.editor.panel.util.AbstractPanelController$6.changed(AbstractPanelController.java:245)
at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:347)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.IntegerPropertyBase.fireValueChangedEvent(IntegerPropertyBase.java:106)
at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:113)
at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:146)
at com.oracle.javafx.scenebuilder.kit.editor.selection.Selection.incrementRevision(Selection.java:532)
at com.oracle.javafx.scenebuilder.kit.editor.selection.Selection.endUpdate(Selection.java:480)
at com.oracle.javafx.scenebuilder.kit.editor.job.v2.CompositeJob.execute(CompositeJob.java:104)
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.panel.inspector.InspectorPanelController.pushJob(InspectorPanelController.java:1207)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController.setSelectionToggleGroup(InspectorPanelController.java:1202)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController.updateValueInModel(InspectorPanelController.java:1128)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController.access$800(InspectorPanelController.java:161)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController$6.changed(InspectorPanelController.java:1094)
at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:176)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(ObjectPropertyBase.java:105)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:145)
at javafx.beans.property.ObjectProperty.setValue(ObjectProperty.java:69)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor.userUpdateValueProperty(PropertyEditor.java:492)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor.userUpdateValueProperty(PropertyEditor.java:476)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.ToggleGroupEditor$1.handle(ToggleGroupEditor.java:72)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.ToggleGroupEditor$1.handle(ToggleGroupEditor.java:56)