Render null class on StateEditor

[mattak]

When state's some property is null, then StateEditor outputs error message. So avoid error and show some help message when state is null.

[mattak]

Memo

https://github.com/mattak/Unidux/blob/master/Assets/Plugins/Unidux/Scripts/Experimental/Editor/UniduxPanelStateTab.cs#L322

                else if (element == null)
                {
                    EditorGUILayout.HelpBox(new StringBuilder(name).Append(" is null").ToString(),
                        MessageType.Info);
                }

[mattak]

resolved by #120