Fix GCC error: control reaches end of non-void function [-Werror=return-type]

--- src/attributeitem.cpp

+++ src/attributeitem.cpp

@@ -87,6 +87,7 @@ QString AttributeItem::getAttributeTypeString()

         case Undefined:

             return "Undefined";

         }

+    return "Undefined";

     }

     void AttributeItem::setInternal(bool b) { internal = b; }