Problem

When an object is not found with objectForPrimaryKey, undefined is returned. This is syntactically not correct. It should return a null value, since the object does not exist. undefined should be reserved for unset properties.

Solution

Change the implementation of objectForPrimaryKey to return null.
Update the types as follows:

objectForPrimaryKey<T>(type: string, key: Realm.PrimaryKey): (T & Realm.Object) | null;
objectForPrimaryKey<T extends Realm.Object>(type: {new(...arg: any[]): T; }, key: Realm.PrimaryKey): T | null;

How important is this improvement for you?

I'd like to see it, but have a workaround