RFR: 8265835: Exception in Quantum due to null platformWindow

[Lukasz Kostyra]

Reproduction code was not provided, I wasn't able to reproduce the circumstances in which the issue occurs. On our side it seems like `WindowStage.getPlatformWindow()` will not return null, however it is also difficult to reject the possibility that some custom user code might trigger an edge-case where a null will be returned.

Adding these checks is not an intrusive change to our code, it is in fact an extra null-check added to already existing checks, but it might prevent the exception from appearing in the future.

I checked other places in our code using `WindowStage.getPlatformWindow()` and none require a null check.

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Commit messages:
 - JDK-8265835: Exception in Quantum due to null platformWindow

Changes: https://git.openjdk.org/jfx/pull/935/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=935&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8265835
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/935.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/935/head:pull/935

PR: https://git.openjdk.org/jfx/pull/935