How to store data in a tablemodel

[spirit]

But the function is getting called with index.row() = -1. Why gets the function called with index.row() = -1?
I still get the same error. What am I doing wrong?

Because an index can be invalid see QModelIndex::isValid.
So, you should at first check the index validity

...
if (!index.isValid())
    return 0;
...

...
if (!index.isValid())
    return 0;
...

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