function onClick(event, instance) {
  if (!isCheckboxInput(event.target)) {
    return false;
  }

  var row = parseInt(event.target.getAttribute('data-row'), 10);
  var col = parseInt(event.target.getAttribute('data-col'), 10);
  var cellProperties = instance.getCellMeta(row, col);

  if (cellProperties.readOnly) {
    event.preventDefault();
  }
}
`click` callback.

@private
@param {Event} event `click` event.
@param {Object} instance Handsontable instance.