I was working on a project that has been developed by another developer which was using your asset. That was a clever move to add the issuetracker link to comment.

Here I`ve corrected it. OnValidate is not designed to be used like that. Your there are many other design flaws and performance issues in the code so beware. Sadly I don`t have time to fix them. However if you are interested you can reach me: poyraz_goksel@hotmail.com

private bool isValidatedInScene;

public virtual void OnValidate()
{
// OnValidate can get called before awake in editor, so make sure the required things are initialized
if( rnd == null ) rnd = GetComponent<MeshRenderer>(); // Needed for ApplyProperties
if( mf == null ) mf = GetComponent<MeshFilter>(); // Needed for UpdateMesh
ShapeClampRanges();
UpdateAllMaterialProperties();
ApplyProperties();
if (isValidatedInScene == false)
{
EditorSceneManager.sceneOpened += OnSceneOpened;
}

// if( MeshUpdateMode == MeshUpdateMode.SelfGenerated )
// meshOutOfDate = true; // we can't update the mesh in OnValidate because of https://issuetracker.unity3d.com/issues/warning-appears-when-changing-meshfilter-dot-sharedmesh-during-onvalidate
// UpdateMesh( force:true ); gosh I wish I could do this it would solve so many problems but Unity has some WEIRD quirks here
isValidatedInScene = true;

}

private void OnSceneOpened(Scene scene, OpenSceneMode mode)
{
EditorSceneManager.sceneOpened -= OnSceneOpened;
UpdateMesh(true);
}

good one <a href="https://www.lyricsauto.com">lyricsauto</a>

Works without any problem with Unity 5.6.X
Warning with 2017.4.5f1

When this issue will get resolved
Will this effect the performance of game?

"By Design" the error message is misleading

Why is this "By Design"?

The warning message has nothing to do with what is causing it. If this really is caused by editing sharedMesh, then the wording of the warning should reflect that.

Same problem if I try to change Layer from OnValidate:
void OnValidate () { gameObject.layer = LayerMask.NameToLayer("Water"); }

Works without any problem with Unity 5.6.X
Warning with 2017.1.0f3 and 2017.1.1f1

I just encountered this. Why is it by design, when SendMessage is not even used (in code)?

Same problem for me for a simple SetActive on a child (always on a "OnValidate" command)

Just by the user operating the Inspector, just creating a new game object with a script from the OnValidate () call will produce a yellow error.

SendMessage cannot be called during Awake, CheckConsistency, or OnValidate