Net dirver already exists

Hey I have a problem:
I tried to make an online game but when i press host, it says:
net dirver already exists.

I already added [OnlineSubsystem] DefaultPlatformService=Null on DefaultEngine.ini file. The error i get is:

Well, I had the same problem but when I cheked my nodes i realised i already created a session somewhere before. thats what the already exists meant.