Strange test failures: validation of PGGroup children failed

[Jeanette Winzenburg]

Just stumbled across it while reviewing PR 719  
(https://github.com/openjdk/jfx/pull/716)

- added the new test method to see it fail (which it does)
- run all control tests throws the error below in _unrelated_ tests  
(the one below is from TableCellTest, that is the same test the new  
failing method resides in, but seeing similar in other tests that use  
Toolkit.firePulse)

Still happens after a clean build from gradle (clean, sdk, :controls:test).

Any idea what might be the reason?

java.lang.AssertionError: validation of PGGroup children failed
	at  
javafx.graphics at 19-internal/javafx.scene.Parent.validatePG(Parent.java:243)
	at  
javafx.graphics at 19-internal/javafx.scene.Parent.doUpdatePeer(Parent.java:201)
	at  
javafx.graphics at 19-internal/javafx.scene.Parent$1.doUpdatePeer(Parent.java:109)
	at  
javafx.graphics at 19-internal/com.sun.javafx.scene.ParentHelper.updatePeerImpl(ParentHelper.java:78)
	at  
javafx.graphics at 19-internal/com.sun.javafx.scene.layout.RegionHelper.updatePeerImpl(RegionHelper.java:72)
	at  
javafx.graphics at 19-internal/com.sun.javafx.scene.NodeHelper.updatePeer(NodeHelper.java:103)
	at javafx.graphics at 19-internal/javafx.scene.Node.syncPeer(Node.java:715)
	at  
javafx.graphics at 19-internal/javafx.scene.Scene$ScenePulseListener.syncAll(Scene.java:2397)
	at  
javafx.graphics at 19-internal/javafx.scene.Scene$ScenePulseListener.syncAll(Scene.java:2406)
	at  
javafx.graphics at 19-internal/javafx.scene.Scene$ScenePulseListener.syncAll(Scene.java:2406)
	at  
javafx.graphics at 19-internal/javafx.scene.Scene$ScenePulseListener.syncAll(Scene.java:2406)
	at  
javafx.graphics at 19-internal/javafx.scene.Scene$ScenePulseListener.synchronizeSceneNodes(Scene.java:2373)
	at  
javafx.graphics at 19-internal/javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2529)
	at  
javafx.graphics at 19-internal/com.sun.javafx.tk.Toolkit.lambda$runPulse$2(Toolkit.java:405)
	at  
java.base/java.security.AccessController.doPrivileged(AccessController.java:389)
	at  
javafx.graphics at 19-internal/com.sun.javafx.tk.Toolkit.runPulse(Toolkit.java:404)
	at  
javafx.graphics at 19-internal/com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:434)
	at  
test.javafx.scene.control.TableCellTest.testEditCancelEventAfterModifyItems(TableCellTest.java:557)