Fix "Please select a SiteTree object to link to"
Fix "Please select a SiteTree object to link to"

On the initial save the SiteTree relation isn't actually saved yet to `->getComponent(...)->exists()` returns false.
While this doesn't validate that it's actually a valid relation ID it does allow you to save.

I'm not sure if this only comes into play when you has_many to Link or if it still affects has_one but it was on has_many when I noticed it.
sheadawson/silverstripe-linkable,sheadawson/silverstripe-linkable