Formulas normally refer to plominoDocument.
When you're looking at the form itself, while you're adding fields and working on the layout, plominoDocument is undefined.
This means that all formulas are raising exceptions.
It's probably also possible to cause unwanted side effects, in the event of formulas that don't use plominoDocument but trigger reindexing or create other documents or whatnot.
It's worthwhile showing syntax errors in formulas to the user, but they shouldn't be executing unless the form is being applied to a document.
