Product Lookup field on Assign Default Value screen of import to custom module non-functional

Product Lookup field on Assign Default Value screen of import to custom module non-functional

All of a sudden the Product Lookup field on a custom module import screen does nothing when I click it.

Steps to reproduce:

1. Create a custom module with a lookup field pointing to a Product. (I don't know if this is specific to a custom module import or any import, as we only use a Product lookup field in a custom module.)
2. Go to the import page for that custom module.
3. The file you import shouldn't matter.
4. On the fields mapping screen, click "Assign Default Value"
5. Under CRM Fields, choose the lookup field you created.
5. Under Default Value, you should be able to click the icon of a box to select the item from a modal window. (Screenshot below.) However when I click this box nothing happens.
Arrow pointing to icon that when clicked should allow you to select a Product from the lookup field.
I have tried this in both Firefox and Chrome.

This just started happening; we import to this custom module every month and this was working fine when we did so about a month ago.

This is a critical part of our business so please let us know a fix or workaround for this issue ASAP.

Thanks.