Why is this segfaulting?

I have the following builder that builds Tokens:

// token_builder.h
class TokenBuilder
{
    Keyword m_keyword;
    TokenType m_type;
    std::string m_symbol;

public:
    TokenBuilder& set_keyword(Keyword);
    TokenBuilder& set_toktype(TokenType);
    TokenBuilder& set_symbol(std::string);
    std::unique_ptr<Token>&& build();
};

// token_builder.cpp
TokenBuilder& TokenBuilder::set_keyword(Keyword k)
  {
      m_keyword = k;
      return *this;
  }

  TokenBuilder& TokenBuilder::set_toktype(TokenType t)
  {
      m_type = t;
      return *this;
  }

  TokenBuilder& TokenBuilder::set_symbol(std::string sym)
  {
      m_symbol = sym; // This is the point where it segfaults
      return *this;
  }

  std::unique_ptr<Token>&& TokenBuilder::build()
  {
      std::unique_ptr<Token> token;
      token->m_keyword = m_keyword;
      token->m_type = m_type;
      token->m_symbol = m_symbol;
      return std::move(token);
  }

The program halts with SIGSEGV at the point where I am trying to initialize m_symbol by calling TokenBuilder::set_symbol. Here is how main looks like:

int main()
{
    auto token = Token::builder()
            .set_keyword(Keyword::IF)
            .set_toktype(TokenType::ID)
            .set_symbol("if")
            .build();

    std::cout << token->get_symbol() << std::endl;
}

As you can see, call to set_keyword and set_toktype don't result in any errors. But passing the literal "if" (or a std::string object) to set_symbol causes the program to crash. Passing by value is intentional. Stepping into set_symbol and analyzing the state of TokenBuilder doesn't reveal anything. As expected, m_keyword is set to Keyword::IF and m_type to TokenType::ID. This is the line that crashes: m_symbol = sym.

The program above might have other, non-trivial issues (am I using std::move correctly? returning to C++ after a long time) but this kind of feels like a silly mistake a rookie would make. I am using MinGW-w64 compiler on Windows 10 (64-bit).

Here is the entire code for conveniently copy pasting to your machine:

// src/include/token_builder.h
#ifndef TOKEN_BUILDER_H
#define TOKEN_BUILDER_H

#include <memory>
#include "token.h"

namespace funk
{
class Token;

class TokenBuilder
{
    Keyword m_keyword;
    TokenType m_type;
    std::string m_symbol;

public:
    TokenBuilder& set_keyword(Keyword);
    TokenBuilder& set_toktype(TokenType);
    TokenBuilder& set_symbol(std::string);
    std::unique_ptr<Token>&& build();
};

}

#endif

// src/token_builder.cpp
#include "include/token.h"
#include "include/token_builder.h"

namespace funk
{
  TokenBuilder& TokenBuilder::set_keyword(Keyword k)
  {
      m_keyword = k;
      return *this;
  }

  TokenBuilder& TokenBuilder::set_toktype(TokenType t)
  {
      m_type = t;
      return *this;
  }

  TokenBuilder& TokenBuilder::set_symbol(std::string sym)
  {
      m_symbol = sym;
      return *this;
  }

  std::unique_ptr<Token>&& TokenBuilder::build()
  {
      std::unique_ptr<Token> token;

      token->m_is_keyword = m_keyword != Keyword::_NONE_;
      token->m_keyword = m_keyword;
      token->m_type = m_type;
      token->m_symbol = m_symbol;

      return std::move(token);
  }
}

// src/include/token.h
#ifndef TOKEN_H
#define TOKEN_H
#include <memory>
#include "keyword.h"
#include "token_type.h"
#include "token_builder.h"

namespace funk
{
  class TokenBuilder;

  class Token
  {
      friend class TokenBuilder;

      Keyword m_keyword;
      bool m_is_keyword;
      std::string m_symbol;
      TokenType m_type;

      Token();

  public:

      Keyword& get_keyword ();
      bool is_keyword();
      std::string& get_symbol();
      TokenType& get_type();
      static TokenBuilder&& builder();
  };
}

#endif // END TOKEN_H

// src/token.cpp
#include <string>
#include <memory>
#include "include/keyword.h"
#include "include/token_builder.h"
#include "include/token_type.h"

namespace funk
{
  Keyword& Token::get_keyword()
  {
    return m_keyword;
  }

  bool Token::is_keyword()
  {
    return m_is_keyword;
  }

  std::string& Token::get_symbol()
  {
    return m_symbol;
  }

  TokenType& Token::get_type()
  {
    return m_type;
  }

  TokenBuilder&& Token::builder()
  {
    TokenBuilder builder;
    return std::move(builder);
  }
}

// src/main.cpp
#include <iostream>
#include "include/buffered_reader.h"
#include "include/token.h"
#include "include/token_builder.h"

int main()
{
    using namespace funk;

    auto token = Token::builder()
            .set_keyword(Keyword::IF)
            .set_toktype(TokenType::ID)
            .set_symbol("if")
            .build();

    std::cout << token->get_symbol() << std::endl;
}

Note: I looked up for similar questions on StackOverflow as best as I could but didn't find one that matches my case. If you think it is similar to a question you previously encountered, feel free to mark duplicate and post the link to the original question. Thanks!