Problem when attempting to change the value of a two-dimensional vector

I am trying to change the values of a two-dimensional QVector (QVector<QVector<QString>). However when attempting it, I get the following error: passing 'const QString' as 'this' argument discards qualifiers [-fpermissive].

I first thought that the problem was in the way I was accessing the data in the QVector. I therefore tried accessing it via operator []. This gave me the following error: no match for 'operator=' (operand types are 'QVector<QString>' and 'QString'). I also tried dereferencing the vector (*SecondList), as I wasn't sure whether operator [] did it and got the following error: no match for 'operator*' (operand type is 'const QVector<QString>') .

Here is the problematic code:

void Handler::changePassword(QString newPassword, int id)
{
    
    QString nP = newPassword;

    int i = 0;
    bool loopBreaker = false;

    while (!loopBreaker)    
    {
        if (SecondList->at(i).at(2) == QString::number(id))
        {//Changes the value for password in "secondlist"
            SecondList->at(i).at(0) = nP;    // ----> ERROR
            loopBreaker = true;
            saveList();
        }
        i++;
    }
}

Here is the header file for Handler class

class Handler : public QWidget
{
private: 
    QVector<QVector<QString>> *SecondList = new QVector<QVector<QString>>;
}

(code is ommitted for readability)

QVector at() returns a const T&, try using operator [] instead, which returns non-const &