java.beans.beancontext.BeanContextSupport.add(Object targetChild)
synchronizes on its argument.

This can be anything, including a type which is a Valhalla value type (eg Integer) and you can't synchronize on a value type.

import java.beans.beancontext.BeanContextSupport;

public class BCT {

    public static void main(String[] argv) {
        BeanContextSupport bcs = new BeanContextSupport();
        bcs.add(Integer.valueOf(1));
    }
}

% java --enable-preview BCT
Exception in thread "main" java.lang.IdentityException: Cannot synchronize on an instance of value class java.lang.Integer
at java.desktop/java.beans.beancontext.BeanContextSupport.add(BeanContextSupport.java:425)
at BCT.main(BCT.java:7)