In the source realm selector, when we select a realm from which we want
to import the data, we pass the source realm's string_id. The problem
with this approach is that the string_id can be an empty string. This
commit makes the source_realm pass the realm's id instead of string_id.
Now, the source_realm's value will either be an integer or "" (empty
string) when we don't want to import settings from any realm.