Error when trying to process rows in a Related field list

[WillowMobileSys]

I have a Service Order Management app that users can place Estimates and then, when approved, convert to an Order. A Work Order object has child references for Products and Materials. When the Estimate is switched to an Order, a Workflow is activated to update Allocated counts in the Inventory table for the associated Products and Materials

I am getting a puzzling error when this Workflow is processed. When I have only a single record in these child references, the updates DO occur as expected in the Inventory table but I still get this error. If there are multiple child records, only the first record is processed and no others. I believe the error is preventing further processing.

What could be causing this error? What am I missing?

Please let me know if you need more information.

Below is the error I am seeing from the log (similar error for each Products and Materials Action sets):

Audit Log Details:

Properties:
{
“AppId”: “ea11df8a-9df1-4f29-afea-d6300774751b”,
“AppTemplateVersion”: “1.000143”,
“RuleName”: “Order Placed”,
“EventType”: “Change”,
“InvokedBy”: “Update”,
“Server”: “produ991100005T”,
“IgnoreSecurityFilters”: false,
“TableName”: “WorkOrders”,
“RuleTableName”: “WorkOrders”,
“OperationUpdateMode”: “UPDATES_ONLY”,
“EventMatch”: “Workflow event successfully matched”,
“Condition”: “=AND([_THISROW_BEFORE].[Work Type] = “Estimate”, [_THISROW_AFTER].[Work Type] = “Order”)”,
“MatchesCondition”: “True”,
“ActionResults”: “Created 1 ActionResults”,
“Action Type”: “TakeAction”,
“Action Name”: “Update Allocation for Materials Used”,
“Errors”: "Error: ‘Action on a set of rows’ ‘Update Materials Allocation’ for table 'Materials failed with exception Value cannot be null.\r\nParameter name: source ",
“AppTemplateName”: “ServiceInsightBase-526414”,
“Operation”: “Workflow action”,
“Result”: “Failure”
}