ADDITIONAL SYSTEM INFORMATION :
os.arch: amd64, Windows 10, OpenJDK 11, OpenJFX 11 SDK

A DESCRIPTION OF THE PROBLEM :
The error occurs if you have embedded JavaFX e.g. in a Eclipse application where each plugin has it's own classloader. When FXPanels are initialized in different Eclipse plugins (classloaders) the following error occurs:

Caused by: java.lang.IllegalStateException: Toolkit already initialized
at javafx.graphics/com.sun.javafx.application.PlatformImpl.startup(PlatformImpl.java:182)
at javafx.graphics/javafx.application.Platform.startup(Platform.java:101)
at javafx.embed.swt.FXCanvas.lambda$initFx$4(FXCanvas.java:354)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:429)
at javafx.embed.swt.FXCanvas.initFx(FXCanvas.java:353)
at javafx.embed.swt.FXCanvas.<clinit>(FXCanvas.java:271)
... 154 more

The Problems was already analysed in the OpenJFX mailing list by Kevin Rushforth. A possible solution was posted, too, see:

http://mail.openjdk.java.net/pipermail/openjfx-dev/2018-October/022610.html

REGRESSION : Last worked in version 11

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create a JavaFX application (e.g. a Eclipse RCP) with different classloaders (e.g. Eclipse plugins) and in the different context initialize a JavaFX panel

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
An exception will be thrown that the toolkit was already initialized.


ACTUAL -
Here the error message of the initialization:

Caused by: java.lang.IllegalStateException: Toolkit already initialized
at javafx.graphics/com.sun.javafx.application.PlatformImpl.startup(PlatformImpl.java:182)
at javafx.graphics/javafx.application.Platform.startup(Platform.java:101)
at javafx.embed.swt.FXCanvas.lambda$initFx$4(FXCanvas.java:354)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:429)
at javafx.embed.swt.FXCanvas.initFx(FXCanvas.java:353)
at javafx.embed.swt.FXCanvas.<clinit>(FXCanvas.java:271)
... 154 more

CUSTOMER SUBMITTED WORKAROUND :
The Problems was already analysed in the OpenJFX mailing list by Kevin Rushforth. A possible solution was posted, too, see:

http://mail.openjdk.java.net/pipermail/openjfx-dev/2018-October/022610.html

FREQUENCY : always