fix for error message when saving lists in the newsletter module

I honestly have no idea if this is the right way to do this. Mind you, I'm only using this field type for the newsletter module. But removing this got rid of "field subscriptions does not exist in table XY" sql error messages. And it didn't make sense for me, since it saves the data to the related table a few lines above.

but as I said: i don't really know this field type, so this PR might be crap :D