[#328] Fix NPE when using ProtocolLib 5.1.0 or older

[Ingrim4]

Description

Fix NPE when using ProtocolLib 5.1.0 or older

Related Issue

closes #328

How Has This Been Tested?

Successfully tested with ProtocolLib 5.1.0 and latest dev build

Types of Changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)