This PR removes the stage from the active windows list after a destroy notification, preventing a possible NPE in `WindowStage::setIconified`.
A system test that fails before the proposed fix, and passes after it, has been included. --------- - [X] I confirm that I make this contribution in accordance with the [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai). ------------- Commit messages: - Remove stage from active windows list, to prevent NPE, and include test Changes: https://git.openjdk.org/jfx/pull/2202/files Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2202&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8387626 Stats: 108 lines in 2 files changed: 107 ins; 0 del; 1 mod Patch: https://git.openjdk.org/jfx/pull/2202.diff Fetch: git fetch https://git.openjdk.org/jfx.git pull/2202/head:pull/2202 PR: https://git.openjdk.org/jfx/pull/2202
