Step to regenerate:
1) Got to Order -> Requirements -> New Requirement
2) On Create Requirement form select Requirement Type Id equals to "Product Requirement"
3) Select any product and submit the form.
4) Here you get the broken screen.
There is a seca rule on "createRequirement" service that triggered "autoAssignRequirementToSupplier" for requirementTypeId equals to "PRODUCT_REQUIREMENT". This service is generating error when quantity is not passed to it. One can easily submit the form to create new requirement without passing quantity as there is not client side validation.