If one try to "reset to default" from the inspector an Enum property, the following assertion is raised.
(e.g. HBox.hgrow)
Seems null value should be handled here.

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.metadata.property.value.EnumerationPropertyMetadata.setValueObject(EnumerationPropertyMetadata.java:142)
at com.oracle.javafx.scenebuilder.kit.editor.job.ModifyObjectJob.redo(ModifyObjectJob.java:87)
at com.oracle.javafx.scenebuilder.kit.editor.job.ModifyObjectJob.execute(ModifyObjectJob.java:74)
at com.oracle.javafx.scenebuilder.kit.editor.job.ModifySelectionJob.execute(ModifySelectionJob.java:78)
at com.oracle.javafx.scenebuilder.kit.editor.JobManager.executeJob(JobManager.java:160)
at com.oracle.javafx.scenebuilder.kit.editor.JobManager.push(JobManager.java:66)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController.pushJob(InspectorPanelController.java:1021)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController.setSelectedFXOMInstances(InspectorPanelController.java:1006)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController.access$1000(InspectorPanelController.java:129)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController$5.changed(InspectorPanelController.java:997)
at com.sun.javafx.binding.ExpressionHelper$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.setValueGeneric(PropertyEditor.java:268)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.EnumEditor.setValue(EnumEditor.java:81)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor$3.handle(PropertyEditor.java:221)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor$3.handle(PropertyEditor.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)