1) open an empty project
2) change the background paint by menu Preview --> Change background color ...
3) quit the tool, a popup is present, select "save", you will get:

Glass detected outstanding Java exception at -[GlassViewDelegate sendJavaLockedEvent]:src/com/sun/mat/ui/GlassViewDelegate.m:872
Exception in thread "AWT-AppKit" java.lang.AssertionError
at com.oracle.javafx.authoring.DesignerTool.saveProject(DesignerTool.java:384)
at com.oracle.javafx.authoring.DesignerTool.canCloseFrame(DesignerTool.java:488)
at com.oracle.javafx.authoring.DesignerTool.exit(DesignerTool.java:515)
at com.oracle.javafx.authoring.MacSceneBuilderLauncher$1.handleQuitAction(MacSceneBuilderLauncher.java:60)
at com.sun.glass.ui.mac.MacApplication$3.action(MacApplication.java:98)