This is reproducible with marina b05. Setting 'icons' to null do not reset the Stage icon.

To reproduce, run the below test. It shows a Stage with a duke image set as Stage icon. On clicking a button on stage, the icons is set to null. This doesn't reset the Stage icon.

import javafx.stage.*;
import javafx.scene.*;
import javafx.scene.image.*;
import javafx.scene.control.*;

var icons: Image[] = [Image {
    url: "http://sqindia.india.sun.com/disk11/jfc/mercurial_workspaces/FX_Test_WS/resources/images/duke.png"
}, Image {
    url: "http://sqindia.india.sun.com/disk11/jfc/mercurial_workspaces/FX_Test_WS/resources/images/duke.jpg"
}];
Stage {
    width: 200
    height: 200
    icons: bind icons
    scene: Scene {
        content: Button {
            text: "Null Icons"
            action: function() {
                //delete icons;
                icons = null;
            }
        }
    }
}