Well, I was doing some code of the proposed exercises. When I went to run it throws the following error:
java.lang.IllegalStateException: Toolkit not initialized
at com.sun.javafx.application.PlatformImpl.runLater(PlatformImpl.java:273)
at com.sun.javafx.application.PlatformImpl.runLater(PlatformImpl.java:268)
at javafx.application.Platform.runLater(Platform.java:83)
at bluej.runtime.ExecServer.runOnTargetThread(ExecServer.java:902)
at bluej.runtime.ExecServer.access$700(ExecServer.java:78)
at bluej.runtime.ExecServer$3.run(ExecServer.java:767)
and now whatever program you try to run gives this error.