fix dupe glitch with backpacks

[J3fftw1]

Description

This PR fixes a dupe glitch with backpacks. There was a race condition in PlayerProfile where the UUID got removed from the loading map before it was finished loading and inserted into the Registry. This meant that the PlayerProfile could load twice if someone did something like spam open a backpack after joining.
We now finish loading and adding to the registry before marking it as no longer loading.

Proposed changes

Move where we finish loading (remove from the loading map) to ensure we're always after the registry.

Related Issues (if applicable)

Fixes #4130
possibly fix for #4131

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16.* - 1.20.*).
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

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[github-actions]

Slimefun preview build

A Slimefun preview build is available for testing!
Commit: 992a1229

https://preview-builds.walshy.dev/download/Slimefun/4134/992a1229

Note: This is not a supported build and is only here for the purposes of testing.
Do not run this on a live server and do not report bugs anywhere but this PR!