FULL PRODUCT VERSION :
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
16.6.0 Darwin Kernel Version 16.6.0: Fri Apr 14 16:21:16 PDT 2017; root:xnu-3789.60.24~6/RELEASE_X86_64 x86_64

A DESCRIPTION OF THE PROBLEM :
When using an accessibility app like e.g. PhraseExpress in combination with a Combobox on screen, I get a NullPointerException each time the value of the combobox changes.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Download and install PhraseExpress for mac (http://www.phraseexpress.com/) - Free to use for 10 phrases
2. Launch PhraseExpress
3. Open the test app
4. Change the value of the combobox

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Value of the combobox changes
ACTUAL -
Value of the combobox changes + NullPointerException

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at com.sun.javafx.scene.control.skin.ComboBoxListViewSkin.queryAccessibleAttribute(ComboBoxListViewSkin.java:593)
at javafx.scene.control.Control.queryAccessibleAttribute(Control.java:917)
at javafx.scene.control.ComboBoxBase.queryAccessibleAttribute(ComboBoxBase.java:459)
at javafx.scene.control.ComboBox.queryAccessibleAttribute(ComboBox.java:616)
at javafx.scene.Node$19.getAttribute(Node.java:9648)
at com.sun.glass.ui.Accessible$GetAttribute.run(Accessible.java:142)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.glass.ui.Accessible.lambda$getAttribute$4(Accessible.java:169)
at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
at com.sun.glass.ui.Accessible.getAttribute(Accessible.java:166)
at com.sun.glass.ui.mac.MacAccessible.accessibilityAttributeValue(MacAccessible.java:1261)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class TestCase extends Application {

@Override
public void start(final Stage primaryStage) throws Exception {
final ComboBox<String> comboBox = new ComboBox<>();
comboBox.getItems().setAll("a", "b", "c");

final Scene scene = new Scene(new StackPane(comboBox), 400, 200);
primaryStage.setScene(scene);
primaryStage.show();
}

}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use a custom skin to fix the broken code:

public static class CustomComboBoxListViewSkin<T> extends ComboBoxListViewSkin<T> {

/***************************************************************************
* * Constructors * *
*************************************************************************
* @param comboBox
*/
public CustomComboBoxListViewSkin(final ComboBox<T> comboBox) {
super(comboBox);
}

@Override
public Object queryAccessibleAttribute(final AccessibleAttribute attribute, final Object... parameters) {
switch (attribute) {
case SELECTION_START:
return getSkinnable().isEditable() ? getEditor().getSelection().getStart() : null;
case SELECTION_END:
return getSkinnable().isEditable() ? getEditor().getSelection().getEnd() : null;
default:
return super.queryAccessibleAttribute(attribute, parameters);
}
}
}