Fix addToolbox() crash (null pointer/reference): create dummy QIcon for overload without icon.

Core/scripting.cc

@@ -297,7 +297,7 @@ void Core::activateToolbox(QString _pluginName, QString _toolboxName, bool activ
 }
 
 void Core::addToolbox(const QString& _name ,QWidget* _widget) {
-    addToolbox(_name, _widget, 0, 0);
+    addToolbox(_name, _widget, new QIcon(), 0);
 }
 
 void Core::addToolbox(const QString& _name ,QWidget* _widget, QIcon* _icon) {