NPE may potentially occur in src/share/classes/javax/swing/plaf/basic/BasicSplitPaneUI.java:

        Integer dividerSize = (Integer)UIManager.get("SplitPane.dividerSize");
check for null >>
        if (divider == null) dividerSize = 10;
        LookAndFeel.installProperty(splitPane, "dividerSize", dividerSize);
dereference without null-check>>
        divider.setDividerSize(splitPane.getDividerSize());
        dividerSize = divider.getDividerSize();

I don't have a test representing that defect but this code was introduced with Nimbus integration so the test might ought to use the Nimbus laf.