BoM formulas from deleted cross-reference could be applied incorrectly

If there is a cross-reference from a BoM line to another BoM, the formulas also reference the parameters of the other BoM. If the cross-reference was removed while it was filled, this formula was - rightly so - no longer visible on the 'Formulas' tab. However, the formula was still present under water and could therefore still be used in some cases. We have improved this.