Grammar Cells OptionalCell gives compiler error when using a projection or only a constant cell

[eugenschindler]

If I use this: or this: or a combination of the two, I get compiler errors. If I use this: it nicely builds. Although obvious for people who know it, I think it would be helpful to add a descriptive error message that tells new users that you cannot use a projection or a constant cell only in this case. For example code, please see attached zip. GrammarCellsTest.zip