OpenJFX: segmentation fault in JavaThread "GtkNativeMainLoopThread"

ADDITIONAL SYSTEM INFORMATION :
OpenJDK 11 64bit, Debian GNU/Linux, OpenJFX 11

A DESCRIPTION OF THE PROBLEM :
PDFsam, a JavaFX application, crashes with a segmentation fault in "GtkNativeMainLoopThread" . This issue can be observed when passing -Djdk.gtk.version=3. It can be worked around by using -Djdk.gtk.version=2. This happens on an up-to-date Debian/GNU Linux system running Debian unstable with OpenJDK 11 and OpenJFX 11.

REGRESSION : Last worked in version 8u192

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
To reproduce install pdfsam from Debian unstable and update /usr/bin/pdfsam to use the -Djdk.gtk.version=3 option. This issue is also reproducible with Oracle JDK 10 and the OpenJFX 11 Maven artifacts from Maven Central.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
PDFsam works with -Djdk.gtk.version=3 like in previous versions with OpenJFX 8.
ACTUAL -
Segmentation fault and core dump.

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apt install pdfsam or apt source pdfsam for sources. https://tracker.debian.org/pdfsam
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CUSTOMER SUBMITTED WORKAROUND :
Use -Djdk.gtk.version=2

FREQUENCY : always