The base class should be a QDialog, not a QWidget. Please set a layout also such that the listview resizes properly when the dialog is resized, and use a QDialogButtonBox with proper button roles to ensure that e.g. the left/right order of Ok/Cancel is adjusted according to what the desktop environment suggests.

Please use Q_UNUSED(parent)

int row = index.row();

This has no effect. You probably want
icon = icon.scaled( ....

This seems to be done in the wrong order: insertRows() above already calls endInsertRows(); which signalizes the attached views that the insertion has finished and they update. In reality though the data is inserted just afterwards in this and the next line.

Q_UNUSED here also

This is dangerous now: addIcon() calls endInsertRows() and relies on insertRows() to call beginInsertRows(). If insertRows() -- which is virtual -- was called by someone else, there would only be an endInsertRows call without a beginInsertRows call before. The Qt docs say

"If you implement your own model, you can reimplement this function if you want to support insertions. Alternatively, you can provide your own API for altering the data. In either case, you will need to call beginInsertRows() and endInsertRows() to notify other components that the model has changed."

We have a mixture of both now that is not consistent. I'd suggest to remove insertRows() and do all that it currently does inside addIcon.