If a Parent has a stylesheet and the stylesheet is not valid (file not found, for example) getStyleHelper will throw an NPE.

java.lang.NullPointerException
at com.sun.javafx.css.StyleManager$StylesheetContainer.getStyleHelper(StyleManager.java:1239)
at com.sun.javafx.css.StyleManager$StylesheetContainer.access$1300(StyleManager.java:1004)
at com.sun.javafx.css.StyleManager.getStyleHelper(StyleManager.java:941)
at javafx.scene.Node.impl_createStyleHelper(Node.java:7359)
at javafx.scene.Node.impl_processCSS(Node.java:7329)