TitledPane fails to initialize in a background thread when adding a series, throwing a
java.lang.NullPointerException: Cannot invoke "javafx.scene.Parent.childVisibilityChanged(javafx.scene.Node)" because the return value of "javafx.scene.Node.getParent()" is null


To reproduce, use the newly developed NodeInitializationStressTest:

```
    @Test
    public void titledPane() {
        assumeFalse(SKIP_TEST);
        test(() -> {
            TitledPane c = new TitledPane("TitledPane", null);
            c.setSkin(new TitledPaneSkin(c));
            return c;
        }, (c) -> {
            c.setAnimated(nextBoolean());
            c.setExpanded(nextBoolean());
            c.setContent(new Label(nextString()));
            accessControl(c);
        });
    }
```

Exception:

java.lang.NullPointerException: Cannot invoke "javafx.scene.Parent.childVisibilityChanged(javafx.scene.Node)" because the return value of "javafx.scene.Node.getParent()" is null
at javafx.graphics/javafx.scene.Node$9.invalidated(Node.java:1441)
at javafx.base/javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:110)
at javafx.base/javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:145)
at javafx.graphics/javafx.css.StyleableBooleanProperty.set(StyleableBooleanProperty.java:103)
at javafx.graphics/javafx.scene.Node.setVisible(Node.java:1420)
at javafx.controls/javafx.scene.control.skin.TitledPaneSkin.lambda$10(TitledPaneSkin.java:396)
at javafx.graphics/com.sun.scenario.animation.shared.TimelineClipCore.visitKeyFrame(TimelineClipCore.java:239)
at javafx.graphics/com.sun.scenario.animation.shared.TimelineClipCore.playTo(TimelineClipCore.java:180)
at javafx.graphics/javafx.animation.Timeline.doPlayTo(Timeline.java:172)
at javafx.graphics/javafx.animation.AnimationAccessorImpl.playTo(AnimationAccessorImpl.java:39)
at javafx.graphics/com.sun.scenario.animation.shared.SingleLoopClipEnvelope.timePulse(SingleLoopClipEnvelope.java:103)
at javafx.graphics/javafx.animation.Animation.doTimePulse(Animation.java:1234)
at javafx.graphics/javafx.animation.Animation$1.timePulse(Animation.java:190)
at javafx.graphics/com.sun.scenario.animation.AbstractPrimaryTimer.timePulseImpl(AbstractPrimaryTimer.java:316)
at javafx.graphics/com.sun.scenario.animation.AbstractPrimaryTimer$MainLoop.run(AbstractPrimaryTimer.java:239)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:588)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:572)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulseFromQueue(QuantumToolkit.java:565)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.lambda$6(QuantumToolkit.java:346)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)


## Root Cause

Animation was started in the background thread, causing concurrent access.

## Solution

Disable animation if not the fx app thread.