git: openjdk/jfx-sandbox: glass_gtk_bug_squashing: Improve testing + fix NPE

[duke]

Changeset: 4c38f00b
Branch: glass_gtk_bug_squashing
Author:    Thiago Milczarek Sayao <thiago.sayao at gmail.com>
Date:      2025-04-08 12:28:05 +0000
URL:       https://git.openjdk.org/jfx-sandbox/commit/4c38f00ba39480067dce2f3a396fd8895a91a455

Improve testing + fix NPE

! modules/javafx.graphics/src/main/native-glass/gtk/glass_window.cpp
! tests/system/src/test/java/test/javafx/stage/StageTestBase.java