basic_string::_M_construct null 在构造字符串的子向量后无效

我的代码应该在一个文本文件中读取，并让多个线程在不同的行块中查看最长的回文。块的大小（多少行）由作为参数传入的可变数量的线程决定。原始文本文件存储在 std：：vector 中，其中矢量的每个索引对应于原始文件。

当我将子向量块传递给 findPalindome（） 时，我得到一个"C++ basic_string：：_M_construct null 无效"，我不知道为什么。我的字符串都不应该为 NULL。

当我传递原始向量线时，我没有收到任何错误，所以我假设这与我创建子向量的方式有关。

这是我的代码：

Result longestPalindrome(std::string str)
{
    int maxLength = 1;  // The result (length of LPS)
    int start = 0;
    int len = str.size();
    int low, high;
    // One by one consider every character as center point of 
    // even and length palindromes
    for (int i = 1; i < len; ++i)
    {
        // Find the longest even length palindrome with center points
        // as i-1 and i.  
        low = i - 1;
        high = i;
        while (low >= 0 && high < len && str[low] == str[high])
        {
            if (high - low + 1 > maxLength)
            {
                start = low;
                maxLength = high - low + 1;
            }
            --low;
            ++high;
        }
        // Find the longest odd length palindrome with center 
        // point as i
        low = i - 1;
        high = i + 1;
        while (low >= 0 && high < len && str[low] == str[high])
        {
             if (high - low + 1 > maxLength)
             {
                start = low;
                maxLength = high - low + 1;
             }
             --low;
            ++high;
        }
    }
    Result result = {0, 0, 0};
    return result;
}
void findPalindome(std::vector<std::string> chunk, Result& result)
{
    Result localLargest = {0,0,0};
    for (unsigned int i = 0; i < chunk.size(); i++)
    {
        Result loopLargest = longestPalindrome(chunk[i]);
        if (localLargest < loopLargest)
        {
            localLargest = loopLargest;
        }
    }
    result = localLargest;
}
Result
FindPalindromeStatic(Lines const& lines, int numThreads)
{
    std::vector<Result> results(numThreads, {0,0,0});;
    int chunkSize = lines.size() / numThreads; //lines is the original vector with all the lines in the file
    std::vector<std::thread> threads;
    int counter = 0;
    for (int i = 0; i < numThreads; i++)
    {
        std::vector<std::string>::const_iterator begin = lines.begin() + counter;
        std::vector<std::string>::const_iterator end = lines.begin() + ((i + 1) * chunkSize);
        std::vector<std::string> chunk(begin, end);
        threads.emplace_back(&findPalindome, std::ref(chunk), std::ref(results[i]));
        counter = ((i+1)*chunkSize);
    }
    for (int i = 0; i < numThreads; i++)
    {
        threads[i].join();
    }
    Result x = {0,0,0};
    return x;
}

帮助将不胜感激，这是我的第一个堆栈问题，因此对任何错误表示歉意。