Unspawn + AdditiveSceneLoad force stops play mode when in Unity Editor.

[iFlex]

Describe the bug
Let's say you play your application in the editor. If it calls NetworkServer.UnSpawn(instance); and then some time later loads an additive scene, Mirror will force stop the playback of the application.

[IMPORTANT] How can we reproduce the issue, step by step:
Use the MirrorTopDownShooter example:

1. Create a new scene without a camera and name it 'test'. In this scene I have a Canvas with an EventSystem, but I think any empty GameObject in there will do as well (maybe even no game object, we just need Unity to load this scene additively)
2. Add the new test scene to the build list. File -> Build Profiles -> Scene List -> Add Open Scenes.
3. Open MirrorTopDownShooter scene again.
4. In MirrorTopDownShooter create an empty GameObject and add a new script to it. I called mine "Reproducer" with the following code:

using Mirror;
using UnityEngine;
using UnityEngine.SceneManagement;

public class Reproducer : MonoBehaviour
{
    [SerializeField] GameObject spawnThis;
    [SerializeField] string sceneName = "test";
    [SerializeField] private GameObject instance;
    
    // Update is called once per frame
    void Update()
    {
        if (Input.GetKeyDown(KeyCode.Alpha1))
        {
            Setup();
        }
        if (Input.GetKeyDown(KeyCode.Alpha2))
        {
            Reproduce();
        }
    }

    void Setup()
    {
        instance = Instantiate(spawnThis, Vector3.zero, Quaternion.identity);
        NetworkServer.Spawn(instance);
    }
    
    void Reproduce()
    {
        NetworkServer.UnSpawn(instance);
        SceneManager.LoadSceneAsync(sceneName, LoadSceneMode.Additive);
    }
}

5. In the Editor, on the Reproducer object, link the SpawnThis variable to EnemyPrefab from MirrorTopDownShooter's prefabs.
6. On the same object, set SceneName to 'test'. Ok we're ready to see it happen.
7. Start the application (press Play in the editor, we're still in MirrorTopDownShooter scene)
8. Once the application starts, click "Player Host".
9. Now on the keyboard press 1. This will spawn an enemy (confirm that by looking at the object hierarchy)
10. Now on the keyboard press 2. This will despawn the enemy and load the additive scene. Very shortly after the application will stop running suddenly. (confirm by verifying that the application is no longer in play mode).

An exception will be thrown and logged:
Scene Assets/Mirror/Examples/TopDownShooter/Scenes/MirrorTopDownShooter.unity needs to be opened and resaved, because the scene object EnemyPrefab(Clone) has no valid sceneId yet. UnityEngine.Debug:LogError (object) Mirror.NetworkScenePostProcess:OnPostProcessScene () (at Assets/Mirror/Editor/NetworkScenePostProcess.cs:77) UnityEditor.Build.BuildPipelineInterfaces:OnSceneProcess (UnityEngine.SceneManagement.Scene,UnityEditor.Build.Reporting.BuildReport)

Root cause:

Mirror's NetworkScenePostProcess kicks in every time a scene is loaded. It goes through all of the NetworkIdentity objects. If any object has sceneId == 0 && !identity.isClient && !identity.isServer Mirror will log the above error and stop the application.

As far as I can tell this is the mechanism for detecting misconfigured scene objects and notifying the user. However, this also happens if you call NetworkServer.UnSpawn(instance); and then load a scene additively. This is because UnSpawn calls ResetState which sets isServer and isClient both to false, causing the scene post processing to missinterpret the unspawned object as misconfigured.

Expected behavior
Mirror should not stop the application and not log the above error when a spawned prefab gets unspawned followed by an additive scene load.

Desktop (please complete the following information):

• OS: [Windows]
• Build target: [Not relevant, but Windows]
• Unity version: [6000.0.59f2]
• Mirror branch: [96.0.1]

[iFlex]

Attempted fix: #4068

Not sure this is 100% sane though, would love some thoughts

[MrGadget1024]

@miwarnec We shouldn't be throwing that error for runtime instantiated objects, spawned or otherwise, and we really shouldn't be running NetworkScenePostProcess on all scenes either...just the last scene loaded.

[MrGadget1024]

Could probably make the same error just by instantiating the thing without spawning it before loading the additive scene.

[iFlex]

#4066 (comment)

It does, just reproduced that too