When a swing component has a tool-tip, and user drag it out into the other than its container, you will see the following messages:

java.lang.NullPointerException:
        at java.awt.swing.ToolTipManager.mouseExited(ToolTipManager.java:222)
        at java.awt.AWTEventMulticaster.mouseExited(AWTEventMulticaster.java:250
)
        at java.awt.Component.processMouseEvent(Component.java:2501)
        at java.awt.Component.processEvent(Component.java:2338)
        at java.awt.Container.processEvent(Container.java:889)
        at java.awt.Component.dispatchEventImpl(Component.java:1909)
        at java.awt.Container.dispatchEventImpl(Container.java:934)
        at java.awt.Component.dispatchEvent(Component.java:1824)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:1541
)
        at java.awt.LightweightDispatcher.setMouseTarget(Container.java:1502)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:1466)

        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:1382)
        at java.awt.Container.dispatchEventImpl(Container.java:921)
        at java.awt.Window.dispatchEventImpl(Window.java:517)
        at java.awt.Component.dispatchEvent(Component.java:1824)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:45)

Here is sample code and steps to re-produce it.
1) Click and hold a mouse button on the "test" button.
2) Now move the mouse to the outside of frame.
3) Release the mouse button.


import java.awt.*;
import java.awt.swing.*;

public class Test extends JFrame
{
    JButton testButton, dialogButton;
    int xx, yy;

    public Test(){
        super("Test Suite");

        JButton button = new JButton("test");
        button.setToolTipText("hold mouse button and move it to outside frame");

        getContentPane().add(button);
        setSize(200, 100);
        show();
    }

    public static void main(String args[]){
        Test frame = new Test();
    }
}