toolsButton GUIContent[]

LevelMeshEditor.cs

[CustomEditor(typeof(LevelMesh))]
public class LevelMeshEditor : Editor
{
    private GUIContent selectButton;
    private string selectButtonIconPath = "Assets/Resources/Icons/select.png";
    
    private GUIContent[] toolsButton;

    private void OnEnable()
    {
        Texture2D selectButtonIcon = AssetDatabase.LoadAssetAtPath<Texture2D>(selectButtonIconPath);
        selectButton = new GUIContent(selectButtonIcon);
        
        toolsButton = new GUIContent[] {selectButton};
    }
}