Fix 'Sniper not found' on login

[Aurelien30000]

Overview

Fixes #326

Description

• Do not rely on Bukkit Player#isValid method as it can return false for a logging in player under some unknown circumstances.
• Provide the safe and existing player object to the Sniper when registering and fetch it only when null.
• Force the player instance to be null when leaving to avoid any potential overlap with the former player instance.

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[NotMyFault]

@SecretlyJealous do you mind testing this PR, whether it resolves your error reported, please?

[OneLiteFeather]

I tried to replicate the issue but (un)fortunately I couldn't reproduce. @TheMeinerLP and me, we're looking for the circumstances / reasons why isValid behaves this way sometimes (our guess was Configuation Phase, but we had 1.18.2 in the issue as well).

We should get feedback from the reporters if their issue still happens in this branch / update to be on the safe side

[NotMyFault]

I'll go ahead and merge to the PR due to lack of response of the issue submitter.