Added null checking for provided input in addInventory
Modified class to reflect Double display, instead of Long
Revert "Added null checking for provided input in addInventory"

This reverts commit 8cc6607df19ea8f3f3a082fb2550a636caa312e7.
Added null checking for provided input in addInventory
Modified class to reflect Double display, instead of Long

Code review : Cyrus Venn Quesada