How to Reverse a null object direction linked to another path ?

Hi everyones , i'm trying to arows in after effects ;

after drawing the path and creating a null object (with trace path technique) for tracking the end of the path to add the arrow head to it ,

everything went well but i have the null object going to the opposite direction of  path and there is no reverse path direction like in standard shapes .

How can i reverse the direction of the null object (it's linked with expression to the other path ) ?