This exception must be catches and advertised thanks to the Error property.

java.lang.NullPointerException: URL must not be null
at javafx.scene.image.Image.validateUrl(Image.java:907)
at javafx.scene.image.Image.<init>(Image.java:512)
at com.sun.javafx.scene.layout.region.BackgroundImageConverter.convert(BackgroundImageConverter.java:82)
at com.sun.javafx.scene.layout.region.BackgroundImageConverter.convert(BackgroundImageConverter.java:37)
at com.sun.javafx.css.StyleHelper.lookup(StyleHelper.java:928)
at com.sun.javafx.css.StyleHelper.transitionToState(StyleHelper.java:736)
at javafx.scene.Node.impl_processCSS(Node.java:6925)
at javafx.scene.Parent.impl_processCSS(Parent.java:1137)
at javafx.scene.Parent.impl_processCSS(Parent.java:1144)
at javafx.scene.Parent.impl_processCSS(Parent.java:1144)
at javafx.scene.Parent.impl_processCSS(Parent.java:1144)
at javafx.scene.Parent.impl_processCSS(Parent.java:1144)
at javafx.scene.Parent.impl_processCSS(Parent.java:1144)