Describe the bug
When you use the filterGroupByField method, if there are more than 2 filtered items, the method returns null. If the filtered items are one or two, the method works correctly.

To Reproduce

1. Open this StackBlitz example
2. Open Browser's console(F12)
3. Click on the "ProductName" column menu and select 3 or more checkboxes
4. See the output in the console which is "null"

Expected behavior
The result returned from the filterGroupByField method should be with a structure similar to the below one, no matter how many checkboxes have been selected in the column menu.