Exception when a user with a valid token is deleted.

Symptom:

Create a user account.
Authenticate using oauth and allow access to the oauth client.
Request a refresh token.
Delete the user account.
Attempt to use the refresh token.

Result:

EntityMetadataWrapperException: Unable to get the data property name as the parent data structure is not set. in EntityStructureWrapper->getPropertyValue() (line 438 of /sites/all/modules/entity/includes/entity.wrapper.inc).

As long as clients continue to request refresh token for this token, it will throw exceptions.

Cause:

Exception stack trace points back to:
oauth2_server/Storage::getRefreshToken() line 203:
'user_id' => $token_wrapper->user->name->value(),

When a user account is deleted the rows in the oauth2_server_token table for that user still exit.

The token is fetched (successfully) from the database however the $token_wrapper->user is NULL because the user no longer exists. Exception thrown.