There are two NPEs that can happen when navigating to a ComboBox with empty string, depending on whether it is editable. You can reproduce both exceptions with HelloCombeBox on Windows 10.

To reproduce:

1. Turn on Navigator screen reader
2. Run HelloComboBox (in rt/apps/toys/Hello)
3. Tab to the second row in the first colum
4. Press F4 to open the menu
5. BUG: an NPE exception is thrown [1]
6. press P4 again to dismiss the menu (the exception will happen again)
7. Tab to the first row (or third row) in the second column
8. BUG: a different NPE exception is thrown [2]

These exceptions are new in JDK 9, although it looks more like like a latent bug than a regression (still, I will add the regression keyword, since it doesn't happen in 8u152).


[1] Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
        at javafx.graphics/com.sun.glass.ui.win.WinAccessible.getUnignoredChildren(WinAccessible.java:493)
        at javafx.graphics/com.sun.glass.ui.win.WinAccessible.Navigate(WinAccessible.java:1052)
        at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:189)
        at java.base/java.lang.Thread.run(Thread.java:844)


[2] Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
        at javafx.graphics/com.sun.glass.ui.win.WinAccessible.get_DocumentRange(WinAccessible.java:1367)
        at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:189)
        at java.base/java.lang.Thread.run(Thread.java:844)