I think just adding variable_get and placing the default values instead of NU...

I think just adding variable_get and placing the default values instead of NULL, will either not solve the problem, as the variable would never be set for existing sites. So we can first check if variable_get gives us the extension, else we set it in our code itself, so that the users having existing sites can still edit the extensions list. Also we can keep the hook enable for new installations. Please suggest.