NULL is defined as nullptr on FreeBSD and causes a build failure

For more information see: https://forum.freecadweb.org/viewtopic.php?f=4&t=22897&p=177660#p177660

src/Mod/PartDesign/Gui/TaskFeaturePick.h

@@ -105,7 +105,7 @@ public:
                         const std::vector<TaskFeaturePick::featureStatus> &status,
                         boost::function<bool (std::vector<App::DocumentObject*>)> acceptfunc,
                         boost::function<void (std::vector<App::DocumentObject*>)> workfunc,
-                        boost::function<void (void)> abortfunc = NULL );
+                        boost::function<void (void)> abortfunc = 0 );
     ~TaskDlgFeaturePick();
 
 public: