RFR: JDK-8298104: NPE on synchronizeSceneNodes()

[buedi]

On Wed, 3 May 2023 11:46:50 GMT, buedi <duke at openjdk.org> wrote:

> A null pointer exception occurs on the ScenePulseListener when iterating through the dirty node list.
> A null check is needed on the node before calling node.getScene().
> 
> The error occurs occasionally and causes the application to crash.

Hi John, thank you for this information. I couldn't find any instances of `ConcurrentModificationException`, but about an hour ago before the NPE, we did have an `IllegalStateException`. Could this be the cause of the issue?
![image](https://user-images.githubusercontent.com/5691507/236412428-2279c9ff-8918-4054-bbaa-a3ec292018c9.png) 
Two weeks ago, we fixed the issue by implementing a check to see if the dialog header was being called from a JavaFX thread.
![image](https://user-images.githubusercontent.com/5691507/236413193-a85133c6-273a-4fb4-9fd7-c9c651790697.png)

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PR Comment: https://git.openjdk.org/jfx/pull/1123#issuecomment-1535930576