Cannot set objective function in Gurobi model C++

Hello,

I have tried to set the objective function in my model based on a quadatric expression GRBQuadExpr that i return from a function in different file other than Main.cpp. But when i run the program it terminates with error 20001 "Variable not in model". 

//test.cpp

typedef std::vector<std::pair<float, float>> vec_pair;

namespace PDPTW_Init

{

int vehicles,customers;

vehicles = 10;
customers = 101;

// This is the initialization of d_matrix
GRB_vec2d init_d_matrix(GRBModel& model,vec_pair coords)
    {
        GRB_vec2d c_matrix(customers, std::vector<GRBVar>(customers));

        for (int i = 0; i < customers; i++)
        {
            c_matrix[i][i] = model.addVar(0, GRB_INFINITY, 0, GRB_CONTINUOUS, "dist from x " + std::to_string(i) + " to " + std::to_string(i)); // Distance from self is zero
            for (int j = 0; j < customers && j!=i; j++)
            {

                double dist_i_j = sqrt(pow((coords[i].first - coords[j].first), 2) + pow((coords[i].second - coords[j].second), 2));

                c_matrix[i][j] = model.addVar(0, GRB_INFINITY,
                    dist_i_j, GRB_CONTINUOUS,
                    "dist from x " + std::to_string(i) + " to " + std::to_string(j));
            }
        }

        return c_matrix;
    }

GRBQuadExpr get_objective(std::vector<std::vector<GRBVar>> c_matrix, GRBVar*** x)
    {
        GRBQuadExpr obj = 0.0;

        for (int k = 0; k < vehicles; k++)
        {
            for (int i = 0; i < customers; i++)
            {
                for (int j = 0; j < customers; j++)
                {
                    
                    obj += c_matrix[i][j] * x[i][j][k];
                }
            }
        }
        return obj;
    }
}

// main.cpp

int main(int argc, char* argv[])
{
        GRBEnv env = GRBEnv();
        GRBModel model = GRBModel(env);

        obj = PDPTW_Init::get_objective(d_matrix, x);

        model.setObjective(obj,GRB_MINIMIZE); // Here is the error
}

Thanks in advance.