Steps to reproduce:

- Create a JavaFX Script file:
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import javafx.ui.*;

Frame{
    attribute a: String;
    
    content: Label{ text: "Frame2" }
    visible: true
}


------------------------------------------

- Enable the Preview
  The frame is shown in the separated window

  There is a IllegalAccessException in the IDE log file
java.lang.IllegalAccessException: Class org.netbeans.modules.javafx.preview.CodeManager can not access a member of class
javafxapplication1.Main$1Frame$anon1 with modifiers "public final"
	at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
	at java.lang.reflect.Field.doSecurityCheck(Field.java:960)
	at java.lang.reflect.Field.getFieldAccessor(Field.java:896)
	at java.lang.reflect.Field.get(Field.java:358)
	at org.netbeans.modules.javafx.preview.CodeManager.parseJFrameObj(CodeManager.java:231)