Magento – Notice: unserialize(); Error at offset in Magento 2.2

$connection need to be an instance of \Magento\Framework\DB\Adapter\AdapterInterface, so update your __contruct() like this

public function __construct(
    ...
    \Magento\Framework\DB\Adapter\AdapterInterface  $connection = null,
    ...
) {
    ...
}

The problem is in /vendor/magento/framework/Serialize/Serializer/Json.php there is a function unserialize($string) which gives you a syntax error if the string is already serialized.

There is a workaround - you can check if string is serialized and then use serialize($string). Change unserialize to:

public function unserialize($string)
{
    /* Workaround: serialize first if is serialized */
    if($this->is_serialized($string))
    {
        $string = $this->serialize($string);
    }
    $result = json_decode($string, true);
    if (json_last_error() !== JSON_ERROR_NONE) {
         throw new \InvalidArgumentException('Unable to unserialize value.');

    }
    return $result;
}

and add function to check if string is serialized:

function is_serialized($value, &$result = null)
{
    // Bit of a give away this one
    if (!is_string($value))
    {
        return false;
    }
    // Serialized false, return true. unserialize() returns false on an
    // invalid string or it could return false if the string is serialized
    // false, eliminate that possibility.
    if ($value === 'b:0;')
    {
        $result = false;
        return true;
    }
    $length = strlen($value);
    $end    = '';
    switch ($value[0])
    {
        case 's':
            if ($value[$length - 2] !== '"')
            {
                return false;
            }
        case 'b':
        case 'i':
        case 'd':
            // This looks odd but it is quicker than isset()ing
            $end .= ';';
        case 'a':
        case 'O':
            $end .= '}';
            if ($value[1] !== ':')
            {
                return false;
            }
            switch ($value[2])
            {
                case 0:
                case 1:
                case 2:
                case 3:
                case 4:
                case 5:
                case 6:
                case 7:
                case 8:
                case 9:
                    break;
                default:
                    return false;
            }
        case 'N':
            $end .= ';';
            if ($value[$length - 1] !== $end[0])
            {
                return false;
            }
            break;
        default:
            return false;
    }
    if (($result = @unserialize($value)) === false)
    {
        $result = null;
        return false;
    }
    return true;
}

After save fe. category without problem, You can restore class to default and there wont be such problem in future.