Fix duplicate agent creation when reactivating scene

[nowsprinting]

Fixed two bugs.

Fix duplicate agent creation when reactivating scene

Problem

Duplicate agent creation.

Conditions

1. Load and activate scene-A, creation agent-A
2. Load scene-B additive and activate (scene-A and agent-A remain)
3. Re-activate scene-A

Cause

Agent creation when SceneManager.activeSceneChanged.

Fix

Agent creation when SceneManager.sceneLoaded.

Fix not dispatch observer agent case

Problem

Not dispatch (creation) observer agent.

Conditions

Specify observer agent, but not specify fallback agent.

Cause

Return before creating observer agent when not specified fallback agent.

Fix

Create observer agent even if fallback agent is not specified.

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[Kuniwak]

LGTM! Thanks!