Por que o código dispara uma NullPointerException?

        tableColumnId.setCellValueFactory(new PropertyValueFactory<>("ID"));
        tableColumnNome.setCellValueFactory(new PropertyValueFactory< >("Nome"));
        tableColumnRg.setCellValueFactory(new PropertyValueFactory<>("RG"));
        tableColumnCpf.setCellValueFactory(new PropertyValueFactory<>("CPF"));
        tableColumnSexo.setCellValueFactory(new PropertyValueFactory<>("Sexo"));
        tableColumnDataNascimento.setCellValueFactory(new PropertyValueFactory<>("Data de Nascimento"));
        tableColumnTelefone.setCellValueFactory(new PropertyValueFactory<>("Telefone"));

    public void popularTabela(){
        DatabaseMySQL db = new DatabaseMySQL();
        Connection con = db.conectar();
        PacienteDao pd = new PacienteDao();
        pd.setConnection((com.mysql.jdbc.Connection) con);
        List<Paciente> list = pd.listar();
        tableColumnId.setCellValueFactory(new PropertyValueFactory<>("ID"));
        tableColumnNome.setCellValueFactory(new PropertyValueFactory< >("Nome"));
        tableColumnRg.setCellValueFactory(new PropertyValueFactory<>("RG"));
        tableColumnCpf.setCellValueFactory(new PropertyValueFactory<>("CPF"));
        tableColumnSexo.setCellValueFactory(new PropertyValueFactory<>("Sexo"));
        tableColumnDataNascimento.setCellValueFactory(new PropertyValueFactory<>("Data de Nascimento"));
        tableColumnTelefone.setCellValueFactory(new PropertyValueFactory<>("Telefone"));
        tableColumnGravidade.setCellValueFactory(new PropertyValueFactory<>("Gravidade"));
        pacientes = FXCollections.observableArrayList(list);
        tableViewPaciente.setItems(pacientes);
    }