Requisition non-full supply product Add Product modal has locked but required dropdowns

The Add Product modal on the Non-Full Supply Products tab of the Requisition form has an unusual select form field behavior. Category and Product are required fields, BUT if you have already added both (there are 2) available products onto your requisition form, then when you are in this modal again those select fields are still required but they are locked/inactive so you cannot use them.

Steps to Reproduce:
1. Log in to OpenLMIS as a user who can create and fill out requisitions
2. Navigate to Requisition > Create/Authorize and Proceed into a new or initiated requisition in Family Planning program (for Comfort facility in Apr2017 period)
3. Click Add Product. If you can choose a Category and Product, that means you have not yet added both available products (there are 2 of them) onto this requisition. So add each one by entering a quantity and explanation. Click Add Product to close the modal.
4. Do step 3 over and over again (twice) until you cannot add any more products.
5. ERROR: Click Add Product to open the modal again. Since all of your available non-full products are already on your form, there really are not any more to choose from. Yet the modal opens anyway. The Category and Product fields are Required, but they are also both inactive/locked where the user cannot make a choice. There is no indication to the user about why it is locked or about how to make a choice or what to do.

PROPOSED SOLUTION (from Nick and Brandon):

• If all of the available non-full products are already added onto this requisition, then clicking "Add Product" button will not open the modal at all. Instead, it will display a warning message to alert the user:
  "There are no more products to add. All available non-full supply products have already been added onto this requisition."