Cause : If item conversion quantity is not properly selected or not entered before finish button click. System will show the error ‘Invalid conversion quantity’ or ‘The conversion quantity can not be null or empty please enter valid conversion quantity’ or ‘Null Pointer Exception’ on Purchase Order.
Remedies:
1) User should enter the conversion factor for converting the unit of measure to the standard unit of measure for the specified item. (System allows to enter within a limit which is defined in UOM conversion master)
2) If unit and Alt unit is different then system will set this field value from UOM conversion master else set as 1.
3) User should ensure this value is properly set after the line item is added, before finishing the record.
3) For UOM Master goto SCM->SCM settings -> Masters->Item Measure->Unit of measure onversion.