I want my users to be able to add only one level of groups to their filters. I want to remove the "Add Group" button in the second level toolbar.

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ADMIN EDIT

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For the time being, such a result can be achieved with CSS. You can target and hide the "Add Group" button of the desired level to prevent the user from creating another nested group.

Example: https://blazorrepl.telerik.com/wxEAOXlo50v2ge6k53.