Here is a modification in the context of the sample project we have sent you. You can try to keep the action parameter of the command in a field of the view model class and in the UpdateCallback(..) method you can try to execute the action:
DelegateCommand(async (s) =>
.callbackAction = (Action)s;
UpdateCallback(RestRequest request, RestResponse response,
//update the Data collection
Let us know if this works for you.
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