To reproduce this, run the following in JIGSAW mode (with JIGSAW_HOME set to a JDK 9 jigsaw build with javafx modules).

$ gradle --info -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:test --tests test.robot.javafx.embed.swing.RT30650Test

test.robot.javafx.embed.swing.RT30650Test > test STANDARD_ERROR
    Exception in thread "JavaFX Application Thread" java.lang.BootstrapMethodError: java.lang.IllegalAccessError: class test.robot.javafx.embed.swing.RT30650GUI (in unnamed module @0x7f8ac96d) cannot access class com.sun.javafx.tk.TKPulseListener (in module javafx.graphics) because module javafx.graphics does not export com.sun.javafx.tk to unnamed module @0x7f8ac96d
     at test.robot.javafx.embed.swing.RT30650GUI.lambda$start$2(RT30650GUI.java:98)
     at com.sun.javafx.application.PlatformImpl.lambda$runLater$5(javafx.graphics@9-internal/PlatformImpl.java:315)
     at java.security.AccessController.doPrivileged(java.base@9-internal/Native Method)
     at com.sun.javafx.application.PlatformImpl.lambda$runLater$6(javafx.graphics@9-internal/PlatformImpl.java:314)
     at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(javafx.graphics@9-internal/InvokeLaterDispatcher.java:96)
     at com.sun.glass.ui.gtk.GtkApplication._runLoop(javafx.graphics@9-internal/Native Method)
     at com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$5(javafx.graphics@9-internal/GtkApplication.java:139)
     at java.lang.Thread.run(java.base@9-internal/Thread.java:804)
    Caused by: java.lang.IllegalAccessError: class test.robot.javafx.embed.swing.RT30650GUI (in unnamed module @0x7f8ac96d) cannot access class com.sun.javafx.tk.TKPulseListener (in module javafx.graphics) because module javafx.graphics does not export com.sun.javafx.tk to unnamed module @0x7f8ac96d
     ... 8 more
Gradle Test Executor 47 finished executing tests.

test.robot.javafx.embed.swing.RT30650Test > test FAILED
    java.lang.Exception: test timed out after 10000 milliseconds
        at jdk.internal.misc.Unsafe.park(Native Method)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:190)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:869)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1023)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1332)
        at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:232)
        at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:205)
        at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:148)
        at javafx.application.Application.launch(Application.java:252)
        at test.robot.javafx.embed.swing.RT30650GUI.test(RT30650GUI.java:65)
        at test.robot.javafx.embed.swing.RT30650Test.test(RT30650Test.java:39)