https://github.com/QtCurve/qtcurve-gtk2/issues/3
https://gist.github.com/anonymous/8e6e4ea32dabf03cf630

--- qtcurve-1.8.18/gtk2/style/qtcurve.c
+++ qtcurve-1.8.18/gtk2/style/qtcurve.c
@@ -372,7 +372,7 @@
     } else if (!(qtSettings.app == GTK_APP_JAVA && widget &&
                  GTK_IS_LABEL(widget))) {
         if (state != GTK_STATE_PRELIGHT || opts.crHighlight ||
-            strcmp(detail, "checkbutton") != 0) {
+            ( detail != NULL && strcmp(detail, "checkbutton") != 0 )) {
             parent_class->draw_flat_box(style, window, state, shadow, area,
                                         widget, detail, x, y, width, height);
         }