Drop down validation

hi experts,

             In an app when the drop down field is "NULL" the validations which we have writen for pop up error messages are not getting displayed, whereas when we remove the drop down property and leave the field for manual entry, the pop ups are displaying fine.

            if we use the "drop down" it shows "Uncaught TypeError: Cannot read property 'getText' of null" .

           Is there any alternative solution for these . please do the needful.