In Scene Builder, I build a fxml like this:
<?xml version="1.0" encoding="UTF-8"?>

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

<VBox maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">
<children><MenuBar>
  <menus>
    <Menu mnemonicParsing="false" text="Help">
      <items>
        <MenuItem mnemonicParsing="false" text="About" />
      </items>
    </Menu>
  </menus>
</MenuBar>
</children></VBox>

When I try to add a SeparatorMenuItem to "Help" Menu.
It make SceneBuilder behave strange. Or in other word, it make SceneBuilder not work.
More concrete, when I do that, I just can't use Document or Library panel to choose other control.

Sorry for poor English, I may be missing something.