Start SceneBuilder then drop a Button on the Content and Save the layout.
The content of the file is:

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import javafx.scene.control.*?>

<Button mnemonicParsing="false" text="Button" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" />

Now Close the layout. I'm running on Mac, which means the SceneBuilder process remains live even there's no document opened.

Now open the fxml file with some text editor and change the content to:

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import javafx.scene.control.*?>

<Button mnemonicParsing="false" text="Button" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">
    <properties foo="123" bar="456"/>
</Button>

As soon as you save the updated fxml file from text editor SceneBuilder process throws the AssertionError documented in DTL-6374.
Isn't Close action supposed to release resources associated to the edition of the layout file ?