Notice: Undefined index: configurable_class in link_field_instance_settings_form

[laryn]

On a site I'm doing initial upgrade work on, when I edit the field settings for a pre-existing Link field, I'm seeing this:

Notice: Undefined index: configurable_class in link_field_instance_settings_form() (line 189 of /app/core/modules/link/link.module).

https://github.com/backdrop/backdrop/blob/1.x/core/modules/link/link.module#L189

[laryn]

Not sure how big of a deal it is since the notice goes away as soon as I save the field configuration on the upgraded site, but maybe we can check if it's empty before assigning it as a default value.