NOTE: the original test will be modified as a part of JDK-8349750 to call show() only in the FX application thread.

SplitMenuButton/ MenuButton fail to initialize in a background thread when adding a series, throwing an exception with a java.lang.NullPointerException: Cannot invoke "javafx.scene.Scene.getStylesheets()" because the return value of "javafx.scene.Node.getScene()" is null

To reproduce, use the newly developed NodeInitializationStressTest:

```
    @Test
    public void splitMenuButton() {
        test(() -> {
            SplitMenuButton c = new SplitMenuButton();
            c.setSkin(new SplitMenuButtonSkin(c));
            return c;
        }, (c) -> {
            c.getItems().setAll(new MenuItem("SplitMenuButton"));
            c.setPopupSide(Side.RIGHT);
            accessControl(c);
            c.show();
        });
    }
```

Exception:

java.lang.NullPointerException: Cannot invoke "javafx.scene.Scene.getStylesheets()" because the return value of "javafx.scene.Node.getScene()" is null
at javafx.controls/javafx.scene.control.ContextMenu.show(ContextMenu.java:250)
at javafx.controls/javafx.scene.control.skin.MenuButtonSkinBase.show(MenuButtonSkinBase.java:297)
at javafx.controls/javafx.scene.control.skin.MenuButtonSkinBase.lambda$4(MenuButtonSkinBase.java:163)
at javafx.controls/com.sun.javafx.scene.control.ListenerHelper$4.changed(ListenerHelper.java:212)
at javafx.base/com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:192)
at javafx.base/com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:91)
at javafx.base/javafx.beans.property.ReadOnlyBooleanPropertyBase.fireValueChangedEvent(ReadOnlyBooleanPropertyBase.java:78)
at javafx.base/javafx.beans.property.ReadOnlyBooleanWrapper.fireValueChangedEvent(ReadOnlyBooleanWrapper.java:103)
at javafx.base/javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:111)
at javafx.base/javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:145)
at javafx.controls/javafx.scene.control.MenuButton.setShowing(MenuButton.java:220)
at javafx.controls/javafx.scene.control.MenuButton.show(MenuButton.java:377)
at test.robot.javafx.scene.NodeInitializationBackgroundThreadTest.lambda$48(NodeInitializationBackgroundThreadTest.java:457)
at test.robot.javafx.scene.NodeInitializationBackgroundThreadTest$1.run(NodeInitializationBackgroundThreadTest.java:561)

## Root Cause

The ContextMenu (PopupWindow) cannot be shown in a background thread.

## Solution

Bail out of `show()` if in a background thread.