Linkediting error when having a static unique_ptr [duplicate]

Because I want my unique_ptr to be the same in all my objects, I decided to make it a static field.

After that an error has appeared:

https://pastebin.com/PPe6z0Ub

I don't really know how to fix it, I've tried searching for it online but I couldn't find anything really useful.

Code: DBFeed.h

#pragma once
#include <vector>
#include <string>
#include <memory>

class DBFeed
{
public:
    void CalculatePosts();
    std::vector<std::string> GetPost();
    void ClearFeed();
private:
    static std::unique_ptr<std::vector<std::vector<std::string>>> m_postari;
    static int m_index;
};

DBFeed.cpp

#include "DBFeed.h"
#include "DBLink.h"

int DBFeed::m_index = 0;

void DBFeed::CalculatePosts()
{
    DBFeed::m_postari = std::make_unique<std::vector<std::vector<std::string>>>();
    pqxx::work worker(*DBLink::GetInstance());
    try
    {
        worker.conn().prepare("CountPostsFeed", "SELECT COUNT(id) FROM posts");
        worker.conn().prepare("ReadAllPostsFeed",
            "SELECT message FROM followers RIGHT JOIN posts ON followers.followed = posts.user_id WHERE followers.follower = 51 LIMIT 5 OFFSET" + std::to_string(m_index+=5)
        );
        pqxx::row countPostsResult = worker.exec_prepared1("CountPostsFeed");
        std::string number = countPostsResult.at("count").c_str();
        pqxx::result readAllPostsResult = worker.exec_prepared_n(std::stoi(number), "ReadAllPostsFeed");
        for (const auto& currentPost : readAllPostsResult)
        {
            std::string postID = currentPost.at("id").c_str();
            std::string message = currentPost.at("message").c_str();
            std::string dateCreated = currentPost.at("date_created").c_str();
            DBFeed::m_postari->push_back(std::vector<std::string>{postID, message, dateCreated});
        }
        worker.commit();
    }
    catch (const pqxx::unexpected_rows&)
    {
        throw std::runtime_error("Reading all of an users posts for feed failed");
    }
}

std::vector<std::string> DBFeed::GetPost()
{
    if (m_postari->empty())
    {
        CalculatePosts();
    }
    std::vector<std::string> deReturnat = m_postari->at(0);
    m_postari->erase(m_postari->begin());
    return deReturnat;
}

void DBFeed::ClearFeed()
{
    m_index = 0;
    m_postari.reset();
}

Why is this error happening and how can I fix it?

//Edit:

Fixed it by adding this by changing DBFeed.cpp's first lines as follows:

#include "DBFeed.h"
#include "DBLink.h"

int DBFeed::m_index = 0;
std::unique_ptr<std::vector<std::vector<std::string>>> 
DBFeed::m_postari = std::make_unique<std::vector<std::vector<std::string>>>();

void DBFeed::CalculatePosts()
{
    pqxx::work worker(*DBLink::GetInstance());
....

But I'd still like to get an explanation on why it was happening and why is it fixed now.