Fix crash in RealmObject finalizer due to null _accessor.

[elle-j]

Description

Fixes #3045.

When creating a RealmObject that throws before being initialized, _accessor will be null. When the object was garbage collected, RealmObject's destructor would call _accessor.UnsubscribeFromNotifications() causing a crash due to NullReferenceException.

Solution:
RealmObject's destructor was removed and instead added the call to UnsubscribeFromNotifications() in ManagedAccessor's destructor. (For an UnmanagedAccessor, a call to UnsubscribeFromNotifications() does nothing.)

TODO

• Changelog entry
• Tests (if applicable)

[peppy]

Thanks for the fix, it looks solid.

As an aside, this is also really cool to see – finalizers are not zero overhead and we've seen performance degradation from having thousands-millions of objects with finalizers. This was unavoidable with RealmObject derived models, but looks like it may become a reality with this change (for cases of UnmanagedAccessor, which is beneficial to us as we use the same model in non-managed batch processing).

[elle-j]

Thank you @peppy for providing the necessary details in your ticket so that we could provide a fix quicker!

[LaPeste]

Interesting subtlety.