delphi xe10 - how to Disable one of the item in TRadioGroup?

i have one RadioGroup with 2 item on my form ( i add item during design from RadioGroup property)

i want to disable one radio button but it give "Access Violation" error when i use below code

TRadioButton(RadioGroup.Controls[1]).Enabled := False;

when i inspect RadioGroup in debug it show 

RadioGroup.ControlCount = 0;

RadioGroup.ComponentCount = 0;