This patch makes kfiledialog functions that did not already do so test the value of d->native before referencing d->w. This is important because if d->native is non-null then d->w will be null and referencing it cause an immediate crash. This was the case on Windows builds of okular when the file chooser was opened.
The bug and patch were previously described here: https://bugs.kde.org/show_bug.cgi?id=364086