Under certain input scenarios, you can get NullPointerExceptions generated when you try to drag a slider thumb.

The program below contains instructions on how to replicate the scenario.

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import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.VBox;
import javafx.scene.text.Font;
import javafx.stage.Stage;

public class SliderDragNull extends Application {
    public static void main(String[] args) { launch(args); }
    
    @Override public void start(Stage stage) {
        Label instructions = new Label(
                "Instructions\n" +
                "\n" +
                " 1. Click the combo box button to drop down the combo.\n" +
                " 2. Don't do anything else.\n" +
                " 3. Try to drag the slider thumb.\n" +
                " 4. NullPointerExceptions will be generated.\n" +
                "\n" +
                "Originally tested with: \tJava 8.0.0-ea-b124, Mac OS X 10.9.1\n" +
                "You are testing with: \tJava "
                        + System.getProperty("javafx.runtime.version") + ", "
                        + System.getProperty("os.name") + " "
                        + System.getProperty("os.version")
                        + "\n" +
                "\n"
        );

        final ComboBox<String> combo = new ComboBox<>();
        combo.setValue(Font.getDefault().getFamily());
        combo.getItems().addAll(Font.getFamilies());

        Slider slider = new Slider(50, 300, 100);

        VBox layout = new VBox(10,
                instructions,
                slider,
                combo
        );
        layout.setPadding(new Insets(10));

        stage.setScene(new Scene(layout));
        stage.show();
    }
}

------

The exception stack trace is as below:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at com.sun.javafx.scene.control.skin.SliderSkin$5.handle(SliderSkin.java:139)
at com.sun.javafx.scene.control.skin.SliderSkin$5.handle(SliderSkin.java:135)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3746)
at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3471)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1695)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2486)
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)

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The cause of the exception is that the dragStart variable has not been initialized and is null when the following code from SliderSkin.java tries to utilize it:

        thumb.setOnMouseDragged(new EventHandler<MouseEvent>() {
            @Override public void handle(MouseEvent me) {
                Point2D cur = thumb.localToParent(me.getX(), me.getY());
                double dragPos = (getSkinnable().getOrientation() == Orientation.HORIZONTAL)?
                    cur.getX() - dragStart.getX() : -(cur.getY() - dragStart.getY());
                getBehavior().thumbDragged(me, preDragThumbPos + dragPos / trackLength);
            }
        });

The dragStart variable is supposed to have been initialized in this event handler:

        thumb.setOnMousePressed(new EventHandler<MouseEvent>() {
            @Override public void handle(MouseEvent me) {
                getBehavior().thumbPressed(me, 0.0f);
                dragStart = thumb.localToParent(me.getX(), me.getY());
                preDragThumbPos = (getSkinnable().getValue() - getSkinnable().getMin()) /
                        (getSkinnable().getMax() - getSkinnable().getMin());
            }
        });

But, for some reason, dragStart is not initialized, so the thumb is not receiving a mouse pressed event before it receives a mouse dragged event => but only for a certain combination of UI input (for instance, as in the provided example, by pressing the Combo Button to drop down the combo before trying to slide the slider).