如何解决settext方法中的nullpointerexception？

Question

我正在编码，然后发现一个问题。当我想设置文本时，出现了nullpointerexception。

我已经尝试修复它。但这没有用。第一次可以使用，但是第二次可以使用nullpointerexception。我不知道为什么？

 public void setBalanceText() {
        balanceRepository.updateBalance();

        String total="Total Balance : "+String.valueOf(balanceRepository.sumBalance();
        totalBalance.setText(total);//where the problem starts
    }

  public void updateBalance() {


        String query = "update info set total=(select sum(amount)from profits)-(select sum(amount)from expenses) where id = 1";
        try {
            connect();
            statement = connection.prepareStatement(query);
            statement.execute();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                disconnect();
            } catch (Exception e) {
            }


}
    }

 public double sumBalance() {

        String query = "select total from info ";

           try {
            connect();
            statement = connection.prepareStatement(query);
            resultSet = statement.executeQuery();
            if (resultSet.next()) {
                return resultSet.getDouble("total");
            }
        } catch (IOException | SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                disconnect();
            } catch (SQLException e) {
            }
        }
        return 0.0;
    }

}

错误：

java.lang.NullPointerException
    at az.controller.BalanceController.setBalanceText(BalanceController.java:267)