[Bug]: "UOM" filter on Inventory Quantities crashes with a server error

Description:

On the Inventory Quantities screen (used to review stock quantities), there is a filter option called "UOM" (unit of measure) meant to let you narrow the list down by unit. This only appears when the "Enable UOM" setting is turned on, and selecting it breaks the page instead of filtering.

Steps to Reproduce:

1. Make sure "Enable UOM" is turned on in the product settings.
2. Go to Inventory → Operations → Quantities.
3. Click the filter icon and choose "Add rule".
4. From the list of fields, select "UOM".

Actual Behaviour:

The page shows an "Internal Server Error" instead of letting you pick a unit of measure to filter by.

Expected Behaviour:

Selecting "UOM" should show a dropdown list of units of measure so you can filter the Quantities list by unit, without any error.

Screenshot: