ADDITIONAL SYSTEM INFORMATION :
Ubuntu 18.10 with OpenJFX used through Maven dependency `org.openjfx:javafx-swing`

A DESCRIPTION OF THE PROBLEM :
In order to avoid test failures due to `UnsupportedOperationException` in a headless environment (e.g. Docker based GitLab CI runner) a user might create a `JFXPanel` and catch `UnsupportedOperationException`. If this test is performed the second time, `new JFXPanel()` never returns.

The issue doesn't occur on a desktop system.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run

```
try {
    System.out.println("Checking availability of JavaFX for tests");
    new JFXPanel();
    System.out.println("JavaFX available for tests");
}catch(UnsupportedOperationException ex) {
    //`new JFXPanel()` for JavaFX toolkit initialization causes
    //`java.lang.UnsupportedOperationException: Unable to open DISPLAY`
    //instead of HeadlessException (which is a subclass of
    //UnsupportedOperationException)
    if("Unable to open DISPLAY".equals(ex.getMessage())) {
        System.out.println("UnsupportedOperationException indicates that the test is run on a headless machine, e.g. a CI service");
    } else {
        throw ex;
    }
}
```

in two different test classes.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Both statements `new JFXPanel()` ought to throw `UnsupportedOperationException` or the latter to fail with a verbose exception.
ACTUAL -
The second statement never returns.

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See https://gitlab.com/krichter/javafx-multiple-initialization/tree/jfxpanel for SSCCE and CI results (the master branch contains a solution with `Platform.startup` which can be used in projects using source features <9 and only works in this scenario, but not in other code - I can investigate further if this issue is confirmed and the fix doesn't fix the issue in the other code).
---------- END SOURCE ----------

FREQUENCY : always