Prevent NPE when passing stage without scene to Dialog.initOwner

[Eng-Fouad]

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Progress

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Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jfx.git pull/1606/head:pull/1606
$ git checkout pull/1606

Update a local copy of the PR:
$ git checkout pull/1606
$ git pull https://git.openjdk.org/jfx.git pull/1606/head

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$ git pr checkout 1606

View PR using the GUI difftool:
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https://git.openjdk.org/jfx/pull/1606.diff

[bridgekeeper]

Hi @Eng-Fouad, welcome to this OpenJDK project and thanks for contributing!

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[Eng-Fouad]

/signed

[bridgekeeper]

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