different messages appear if formula field is ticked but value box is left empty (default value is deleted, as default value is "1")
when clicks save on:

-items -page will refresh and error message will appear "Warning. Field "Custom multiplier" is mandatory."
at the same time value "1" (default) will appear in formula field

-item prototype -page will refresh and error message will appear "Warning. Field "Custom multiplier" is not decimal number."
and no default value ("1") will appear in formula field