[Enhancement] Nullcheck of value previously dereferenced

[jjastan]

Before Creating the Enhancement Request

• I have confirmed that this should be classified as an enhancement rather than a bug/feature.

Summary

The method AbstractRocksDBStorage#openget get value from event first, and then do null check for event.
This may cause a NPE.

Motivation

When this.db is null, NPE may be thrown instead of RocksDBException as expected in the following code

Describe the Solution You'd Like

move null check operate forward.

Describe Alternatives You've Considered

No.

Additional Context

No response