code: protected function checkValue($varInput)
  	{
  		if ($varInput == '' || !\is_array($this->rootNodes))
  		{
  			return;
  		}
  
  		if (strpos($varInput, ',') === false)
  		{
  			$arrIds = array((int) $varInput);
  		}
  		else
  		{
  			$arrIds = array_map('\intval', array_filter(explode(',', $varInput)));
  		}
  
  		if (\count(array_diff($arrIds, array_merge($this->rootNodes, $this->Database->getChildRecords($this->rootNodes, 'tl_page')))) > 0)
  		{
  			$this->addError($GLOBALS['TL_LANG']['ERR']['invalidPages']);
  		}
  	}
docs: Check the selected value
  
  @param mixed $varInput