On Ubuntu 16.04, something like 5 times out of 7, when running
"gradle test" with OpenJFX 15 on top of JDK 12 and using
gradle 5.6.3 I get the failure shown below.
Interestingly running JUST the graphics tests I have not yet seen it.
It only occurs when running the graphics tests after the controls, fxml etc tests

> Task :graphics:test

test.javafx.css.Node_cssStyleMap_Test > testStyleMapChildren FAILED
    java.lang.NullPointerException
        at javafx.graphics/javafx.scene.Node.notifyParentsOfInvalidatedCSS(Node.java:9387)
        at javafx.graphics/javafx.scene.Node.reapplyCSS(Node.java:9428)
        at javafx.graphics/javafx.scene.Node$1.reapplyCSS(Node.java:579)
        at javafx.graphics/com.sun.javafx.scene.NodeHelper.reapplyCSS(NodeHelper.java:282)
        at javafx.graphics/com.sun.javafx.css.StyleManager.userAgentStylesheetsChanged(StyleManager.java:1514)
        at javafx.graphics/com.sun.javafx.css.StyleManager.setDefaultUserAgentStylesheet(StyleManager.java:1488)
        at test.javafx.css.Node_cssStyleMap_Test.testStyleMapChildren(Node_cssStyleMap_Test.java:197)

22988 tests completed, 1 failed, 273 skipped

> Task :graphics:test FAILED

FAILURE: Build failed with an exception.