make WStandardItemModel more resilient
The following code will segfault:
auto model = std::make_unique<Wt::WStandardItemModel>(); model->removeColumns(0, model->columnCount());
Yes, the code does not make a lot of sense and the argument could be made that this is user error, but an API should be able to handle this gracefully.
I suggest simply doing nothing in removeColumns/Rows() if the count is zero - or throw an exception at my face :)
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