addStrongReference method
Add a new instance with instanceId as key and instance as the value.
instance is stored as a strong reference.
Returns true if the pair is successfully added. Returns false if
the instanceId or instance is already contained in the manager or the
instance is a num, bool, or String.
Implementation
bool addStrongReference({required Object instance, String? instanceId}) {
if (!_isValidInstance(instance)) return false;
final String newId = instanceId ?? generateUniqueInstanceId(instance);
_instanceIds[instance] = newId;
_strongReferences[newId] = instance;
return true;
}