Insert a FlowPane in content panel.
Click on one of the insets field in the inspector.
Click back on the FlowPane in content panel.

You will get :

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor.isSimpleValueChanged(PropertyEditor.java:499)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor.isValueChanged(PropertyEditor.java:493)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor.userUpdateValueProperty(PropertyEditor.java:467)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.InsetsEditor$1.handle(InsetsEditor.java:92)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.InsetsEditor$1.handle(InsetsEditor.java:84)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController$5$1.call(InspectorPanelController.java:1051)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController$5$1.call(InspectorPanelController.java:1047)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.canGetFxmlText(EditorController.java:271)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.activateGesture(EditModeController.java:844)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.mousePressedOnGlassLayer(EditModeController.java:691)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.access$300(EditModeController.java:93)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController$4.handle(EditModeController.java:501)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController$4.handle(EditModeController.java:498)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3684)
at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3409)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1671)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2462)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:314)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:243)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:345)
at com.sun.glass.ui.View.handleMouseEvent(View.java:526)
at com.sun.glass.ui.View.notifyMouse(View.java:898)
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor.isSimpleValueChanged(PropertyEditor.java:499)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor.isValueChanged(PropertyEditor.java:493)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor.userUpdateValueProperty(PropertyEditor.java:467)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.InsetsEditor$1.handle(InsetsEditor.java:92)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.InsetsEditor$1.handle(InsetsEditor.java:84)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController$5.changed(InspectorPanelController.java:1068)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.InspectorPanelController$5.changed(InspectorPanelController.java:1041)
at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:176)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.BooleanPropertyBase.fireValueChangedEvent(BooleanPropertyBase.java:103)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:110)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:143)
at javafx.beans.property.BooleanProperty.setValue(BooleanProperty.java:77)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor$6.changed(PropertyEditor.java:791)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.PropertyEditor$6.changed(PropertyEditor.java:785)
at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:347)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.ReadOnlyBooleanPropertyBase.fireValueChangedEvent(ReadOnlyBooleanPropertyBase.java:72)
at javafx.scene.Node$FocusedProperty.notifyListeners(Node.java:7486)
at javafx.scene.Node.setFocused(Node.java:7537)
at javafx.scene.Scene$KeyHandler.setWindowFocused(Scene.java:3856)
at javafx.scene.Scene$KeyHandler$1.invalidated(Scene.java:3875)
at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:339)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.ReadOnlyBooleanWrapper$ReadOnlyPropertyImpl.fireValueChangedEvent(ReadOnlyBooleanWrapper.java:178)
at javafx.beans.property.ReadOnlyBooleanWrapper$ReadOnlyPropertyImpl.access$100(ReadOnlyBooleanWrapper.java:148)
at javafx.beans.property.ReadOnlyBooleanWrapper.fireValueChangedEvent(ReadOnlyBooleanWrapper.java:144)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:110)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:143)
at javafx.stage.Window.setFocused(Window.java:409)
at com.sun.javafx.stage.WindowPeerListener.changedFocused(WindowPeerListener.java:59)
at com.sun.javafx.tk.quantum.GlassWindowEventHandler.run(GlassWindowEventHandler.java:89)
at com.sun.javafx.tk.quantum.GlassWindowEventHandler.run(GlassWindowEventHandler.java:39)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassWindowEventHandler.handleWindowEvent(GlassWindowEventHandler.java:130)
at com.sun.glass.ui.Window.handleWindowEvent(Window.java:1235)
at com.sun.glass.ui.Window.notifyFocus(Window.java:1210)