Test Runs Summary breaks down when Grouped By - Assignee

Steps to reproduce:

1. Add the "Test Runs Summary" gadget to a dashboard.
2. Add any filter to the gadget. The filter must include test runs with the assignee "[migration] unknown".
3. In the field Group By, select Assignee.

Expected Result:

The Test Runs included in the filter should be displayed grouped by Assignee, with each Assignee being represented by a column:

Actual Result:

The gadget breaks, displayed the following error: "Error initializing gadget".

Additional Notes:

The gadget works for filters with a small number of Test Runs. However, if the selected filter includes a large number of Test Runs (possibly >50), the gadget will break with the error above.