sample .fxml where sample2.fxml is located in the same directory:

<GridPane alignment="center" hgap="10" vgap="10" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8" fx:controller="sample.Controller">
    <fx:include source="sample2.fxml"/>
</GridPane>



java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.metadata.util.DesignHierarchyMask.getColumnsSize(DesignHierarchyMask.java:766)
at com.oracle.javafx.scenebuilder.kit.metadata.util.DesignHierarchyMask.getClassNameInfo(DesignHierarchyMask.java:234)
at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.HierarchyItem.getClassNameInfo(HierarchyItem.java:264)
at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.treeview.HierarchyTreeCell.updateLayout(HierarchyTreeCell.java:812)
at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.treeview.HierarchyTreeCell.updateItem(HierarchyTreeCell.java:518)
at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.treeview.HierarchyTreeCell.updateItem(HierarchyTreeCell.java:114)
at javafx.scene.control.TreeCell.updateItem(TreeCell.java:509)
at javafx.scene.control.TreeCell.access$700(TreeCell.java:71)
at javafx.scene.control.TreeCell$8.invalidated(TreeCell.java:170)
at javafx.beans.WeakInvalidationListener.invalidated(WeakInvalidationListener.java:83)
at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:339)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.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.scene.control.TreeView.setRoot(TreeView.java:447)
at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.treeview.HierarchyTreeViewController.updatePanel(HierarchyTreeViewController.java:101)
at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.AbstractHierarchyPanelController.fxomDocumentDidChange(AbstractHierarchyPanelController.java:455)
at com.oracle.javafx.scenebuilder.kit.editor.panel.util.AbstractPanelController$1.changed(AbstractPanelController.java:90)
at com.oracle.javafx.scenebuilder.kit.editor.panel.util.AbstractPanelController$1.changed(AbstractPanelController.java:81)
at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:347)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.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.EditorController.updateFxomDocument(EditorController.java:2378)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.setFxmlTextAndLocation(EditorController.java:655)