CheckboxGroup

Answer:
At least one Checkbox in a CheckboxGroup must be selected at all times. A CheckboxGroup mimics a set of radio buttons, where the pressing of a button results in the ejection of all of other buttons. In general, if you require all of your checkboxes to be unselected, it is possible that they really should not be in a CheckboxGroup.

However, if the options represented by the checkboxes are indeed mutually exclusive, and you really would like to be able to deselect all of them, you can use the following trick:

Add an extra Checkbox to the CheckboxGroup, but don't add it to a container component and do not display it. When you want to deselect all the visible checkboxes, just set the invisible Checkbox's state to true. From a user interface standpoint, it may be better to make this extra button visible and give it a label like "None" so that it is clear to the user that it is possible not to choose any of the other options.