[BUG] set_topology causing crash.

[kilasuelika]

Describe the bug
If set the

archipelago archi{ 16u, algo, prob, 20u };
archi.set_topology(pagmo::topology(pagmo::ring()));

then program crash. remove the line ok.

To Reproduce
Code:

#include <iostream>

#include <pagmo/algorithm.hpp>
#include <pagmo/algorithms/sade.hpp>
#include <pagmo/algorithms/pso.hpp>
#include <pagmo/archipelago.hpp>
#include <pagmo/problem.hpp>
#include <pagmo/problems/schwefel.hpp>
#include <pagmo/topologies/ring.hpp>
using namespace pagmo;

struct CustomSphere {
	CustomSphere(unsigned int dim = 30) : m_dim(dim) {}

	std::vector<double> fitness(const std::vector<double>& x) const {
		double sum = 0.0;
		for (const auto& xi : x) {
			sum += xi * xi;
		}
		return { sum };
	}

	std::pair<std::vector<double>, std::vector<double>> get_bounds() const {
		std::vector<double> lb(m_dim, -5.0);
		std::vector<double> ub(m_dim, 5.0);
		return { lb, ub };
	}

	unsigned int get_nobj() const {
		return 1;
	}

	std::string get_name() const {
		return "Custom Sphere Function";
	}

private:
	unsigned int m_dim;
};


int main()
{
	// 1 - Instantiate a pagmo problem constructing it from a UDP
	// (i.e., a user-defined problem, in this case the 30-dimensional
	// generalised Schwefel test function).
	problem prob{ CustomSphere(30) };

	// 2 - Instantiate a pagmo algorithm (self-adaptive differential
	// evolution, 100 generations).
	algorithm algo{ pagmo::pso(100) };

	algo.set_verbosity(5);
	// 3 - Instantiate an archipelago with 16 islands having each 20 individuals.
	archipelago archi{ 16u, algo, prob, 20u };
	archi.set_topology(pagmo::topology(pagmo::ring()));

	// 4 - Run the evolution in parallel on the 16 separate islands 10 times.
	archi.evolve(10);

	// 5 - Wait for the evolutions to finish.
	archi.wait_check();

	// 6 - Print the fitness of the best solution in each island.
	for (const auto& isl : archi) {
		std::cout << isl.get_population().champion_f()[0] << '\n';
	}
}

Expected behavior
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Environment (please complete the following information):

• OS: [e.g. Linux] windows
• Installation method: [e.g. conda, source] vcpkg
• Version: [e.g. 2.15] 2.19.1

Additional context
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