Possible bug in Unity3D?

Question

Apparently after the update of Unity3D I can no longer switch from one scene to another. I do not know if it's a problem due to the update but at the moment I get this error when I click on a button that should send me back to another scene.

ArgumentException: method return type is incompatible
System.Delegate.CreateDelegate (System.Type type, System.Object firstArgument, System.Reflection.MethodInfo method, Boolean throwOnBindFailure) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/Delegate.cs:190)
System.Delegate.CreateDelegate (System.Type type, System.Object firstArgument, System.Reflection.MethodInfo method) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/Delegate.cs:276)
UnityEngineInternal.NetFxCoreExtensions.CreateDelegate (System.Reflection.MethodInfo self, System.Type delegateType, System.Object target) (at /Users/builduser/buildslave/unity/build/Runtime/Export/WinRT/NetFxCoreExtensions.cs:11)
UnityEngine.Events.InvokableCall..ctor (System.Object target, System.Reflection.MethodInfo theFunction) (at /Users/builduser/buildslave/unity/build/Runtime/Export/UnityEvent.cs:149)
UnityEngine.Events.PersistentCall.GetRuntimeCall (UnityEngine.Events.UnityEventBase theEvent) (at /Users/builduser/buildslave/unity/build/Runtime/Export/UnityEvent.cs:447)
UnityEngine.Events.PersistentCallGroup.Initialize (UnityEngine.Events.InvokableCallList invokableList, UnityEngine.Events.UnityEventBase unityEventBase) (at /Users/builduser/buildslave/unity/build/Runtime/Export/UnityEvent.cs:610)
UnityEngine.Events.UnityEventBase.RebuildPersistentCallsIfNeeded () (at /Users/builduser/buildslave/unity/build/Runtime/Export/UnityEvent.cs:776)
UnityEngine.Events.UnityEventBase.PrepareInvoke () (at /Users/builduser/buildslave/unity/build/Runtime/Export/UnityEvent.cs:812)
UnityEngine.Events.UnityEvent.Invoke () (at /Users/builduser/buildslave/unity/build/Runtime/Export/UnityEvent_0.cs:53)
UnityEngine.UI.Button.Press () (at /Users/builduser/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:36)
UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at /Users/builduser/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:45)
UnityEngine.EventSystems.ExecuteEvents.Execute (IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at /Users/builduser/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:50)
UnityEngine.EventSystems.ExecuteEvents.Execute[IPointerClickHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) (at /Users/builduser/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:261)
UnityEngine.EventSystems.EventSystem:Update()

Currently the error happens with the 2018.1.0f2 version and unfortunately I can not remember the version I used previously.

Here is the code relative to the Script responsible of the scene switch, and this works fine:

using UnityEngine.SceneManagement;
using UnityEngine;

public class MainEngine : MonoBehaviour {

    // Use this for initialization
    void Start () {
        DontDestroyOnLoad(this.gameObject);
        SceneManager.activeSceneChanged += ChangedActiveScene;
        SceneManager.LoadScene("Introduction");
    }

    // Update is called once per frame
    void Update () {

    }

    private void ChangedActiveScene(Scene current, Scene next)
    {
        string currentName = current.name;

        if (currentName == null)
        {
            // Scene1 has been removed
            currentName = "Replaced";
        }

        Debug.Log("Scenes: " + currentName + ", " + next.name);
    }
}

But this following is the "guilty" code:

using UnityEngine.SceneManagement;
using UnityEngine;
using System.Collections;

public class InstructionsScene : MonoBehaviour {

    IEnumerator PlayNow()
    {
        Debug.Log("scene should be changed");

        float fadeTime = GameObject.Find("SceneEngine").GetComponent<Fading>().BeginFade(1);
        yield return new WaitForSeconds(fadeTime);
        SceneManager.LoadScene("Instructions", LoadSceneMode.Single);      
    }

}

Answer 1

A callback for a button (possibly UI elements in general) has to be of return type void . Yours is of return type IEnumerator (if PlayNow is the function you call from your button).

A workaround is to start the coroutine from within PlayNow :

public void PlayNow()
{
    Debug.Log("scene should be changed");

    StartCoroutine(PlayNowCoroutine());
}

IEnumerator PlayNowCoroutine()
{
    float fadeTime = GameObject.Find("SceneEngine").GetComponent<Fading>().BeginFade(1);
    yield return new WaitForSeconds(fadeTime);
    SceneManager.LoadScene("Instructions", LoadSceneMode.Single); 
}