Symfony\Component\Debug\Exception\FlattenException::getPrevious PHP Method

    public function getPrevious()
    {
        return $this->previous;
    }

 /**
  * This method is a temporary port of _drupal_decode_exception().
  *
  * @todo This should get refactored. FlattenException could use some
  *   improvement as well.
  *
  * @param \Symfony\Component\Debug\Exception\FlattenException $exception
  *  The flattened exception.
  *
  * @return array
  *   An array of string-substitution tokens for formatting a message about the
  *   exception.
  */
 protected function decodeException(FlattenException $exception)
 {
     $message = $exception->getMessage();
     $backtrace = $exception->getTrace();
     // This value is missing from the stack for some reason in the
     // FlattenException version of the backtrace.
     $backtrace[0]['line'] = $exception->getLine();
     // For database errors, we try to return the initial caller,
     // skipping internal functions of the database layer.
     if (strpos($exception->getClass(), 'DatabaseExceptionWrapper') !== FALSE) {
         // A DatabaseExceptionWrapper exception is actually just a courier for
         // the original PDOException.  It's the stack trace from that exception
         // that we care about.
         $backtrace = $exception->getPrevious()->getTrace();
         $backtrace[0]['line'] = $exception->getLine();
         // The first element in the stack is the call, the second element gives us the caller.
         // We skip calls that occurred in one of the classes of the database layer
         // or in one of its global functions.
         $db_functions = array('db_query', 'db_query_range');
         while (!empty($backtrace[1]) && ($caller = $backtrace[1]) && (strpos($caller['namespace'], 'Drupal\\Core\\Database') !== FALSE || strpos($caller['class'], 'PDO') !== FALSE) || in_array($caller['function'], $db_functions)) {
             // We remove that call.
             array_shift($backtrace);
         }
     }
     $caller = Error::getLastCaller($backtrace);
     return array('%type' => $exception->getClass(), '!message' => String::checkPlain($message), '%function' => $caller['function'], '%file' => $caller['file'], '%line' => $caller['line'], 'severity_level' => WATCHDOG_ERROR);
 }