Relatively self-explanatory, when trying to add a .css file that is converted to binary (and thus a .bss file), javafx throws a NPE.

A basic example of this can be seen with the following code:

MenuBarTest.java:

/**
 *
 * @author William
 */
public class MenuBarTest extends Application {


    @Override
    public void start(Stage stage) throws IOException {
        Parent root = FXMLLoader.load(getClass().getResource("style/testFXML.fxml"));
        root.getStylesheets().add(getClass().getResource("style/test.css").toExternalForm());
        Scene scene = new Scene(root);

        //scene.getStylesheets().add(getClass().getResource("css/Calendar.css").toExternalForm());
       
        //scene.
        stage.setScene(scene);
        // ScenicView.show(scene);
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}


style/test.css:


/*
    Created on : Aug 7, 2014, 12:01:31 PM
    Author : William
*/


.menu .label{
    -fx-text-fill: black;
}

#menuBar
{
    -fx-background-color: linear-gradient(to bottom, #ebeced, #b5bcc6);

}

style/testFXML.fxml:

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

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

<AnchorPane id="AnchorPane" prefHeight="400.0" prefWidth="600.0" xmlns:fx="http://javafx.com/fxml/1">
        <MenuBar id="menuBar" fx:id="menu" depthTest="DISABLE" focusTraversable="true" pickOnBounds="true" prefHeight="24.0" prefWidth="860.0">
            <menus>
                <Menu id="menu_file" text="File" fx:id="menu_file">
                    <items>
                        <MenuItem id="close1" text="Close" />
                    </items>
                </Menu>
                <Menu id="menu_tools" text="Data">
                    <items>
                        <MenuItem id="menuItem1" text="Meta data" />
                    </items>
                </Menu>
                <Menu mnemonicParsing="false" text="Tools">
                    <items>
                        <MenuItem mnemonicParsing="false" text="Read File" />
                    </items>
                </Menu>
                <Menu id="menu_about" text="About">
                    <items>
                        <MenuItem id="cf_f_mi_abt_mag" text="MagVAR" fx:id="cf_f_mi_abt_mag" />
                    </items>
                </Menu>
                <Menu id="menu_help" text="Help">
                    <items>
                        <MenuItem id="menuItem2" text="Read Me" fx:id="cf_menu_hel_red" />
                        <MenuItem id="menuItem3" text="Software Manual" />
                        <MenuItem id="menuItem4" text="Calculator Version" />
                        <MenuItem id="menuItem5" text="Feedback" />
                    </items>
                </Menu>
            </menus>
        </MenuBar>
</AnchorPane>


Using netbeans I simply chose the "Binary Encode JavaFX CSS Files" option under 'Project properties > build > Packaging" . Or you can reproduce this behaviour with the javafx gradle plugin, with runs a 'cssToBin' task by default, and creates some interesting undefined behaviour. stack trace below:

Executing C:\Users\William\Documents\compactJRE\dist\run1706831133\compactJRE.jar using platform C:\Program Files (x86)\Java\jdk1.8.0_20/bin/java
Exception in Application start method
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:363)
at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:303)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.RuntimeException: Exception in Application start method
at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:875)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$147(LauncherImpl.java:157)
at com.sun.javafx.application.LauncherImpl$$Lambda$48/9727497.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
at graphtest.MenuBarTest.start(MenuBarTest.java:20)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$153(LauncherImpl.java:821)
at com.sun.javafx.application.LauncherImpl$$Lambda$51/926275.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$166(PlatformImpl.java:323)
at com.sun.javafx.application.PlatformImpl$$Lambda$44/14208992.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$null$164(PlatformImpl.java:292)
at com.sun.javafx.application.PlatformImpl$$Lambda$47/16186433.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$165(PlatformImpl.java:291)
at com.sun.javafx.application.PlatformImpl$$Lambda$46/31501478.run(Unknown Source)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$141(WinApplication.java:102)
at com.sun.glass.ui.win.WinApplication$$Lambda$37/29531133.run(Unknown Source)
... 1 more
Exception running application graphtest.MenuBarTest
Java Result: 1

obviously a workaround is to simply not use it, but that seems like an unfortunate thing to require.