Gets a reference to the main Unity editor window.

GetMainEditorWindow.cs

static Object GetMainEditorWindow()
{
    var containerWindowType = typeof(EditorWindow).Assembly.GetType("UnityEditor.ContainerWindow");
    var showModeField = containerWindowType.GetField("m_ShowMode", BindingFlags.Instance | BindingFlags.NonPublic);

    return Resources
        .FindObjectsOfTypeAll(containerWindowType)
        .First(window => (int)showModeField.GetValue(window) == 4);
}