Swift Firebase检查是否存在值

I am trying to check if a value exists inside a user. I am creating basically a check to activate or deactivate a button in regards if the value exists. I have a follow/unfollow system but the problem is that if one user has not yet added certain values and a user wants to follow them, the app will crash. So I just want to deactivate the button until the user adds the values which would then make the button active. I have been giving this code below a try but no luck. I want to check if they created a value "snapchatURL", it doesn't matter if it holds a URL or not, I only need to know if that value exists with or without the URL. I am trying to check if a value exists inside a