Wrong default saving location, & beyond that user can not change it !

Hi. I tried this package. I noticed that default saving location when I tried to save newly created PDF is pointed to wrong location which is:

run/user/1000/doc/--------

where ------- is something like "kfdjuh37"

I faced with such issue with pdfslicer & it's developer fix it - see: https://github.com/junrrein/pdfslicer/issues/72

Also, I faced with such issue with PDF Mix - see: https://gitlab.com/scarpetta/pdfmixtool/issues/13

The diffirence with Master PDF Editor is that even when I select other location like /home/myusername/Desktop & save, the document saved to same wrong location !! So, in pdfslicer & PDF Mix it was possible to overcome this issue when user change location to other one. But with Master PDF Editor even this work around not working at all !

まず Flatpak で PDF の保存を再現し、デフォルトのパスと選択した保存先を比較します。この issue と、リンクされている pdfslicer および PDF Mix のレポートに記載された動作を確認します。新しく作成された PDF が run/user/1000/doc/-------- ではなく、選択した場所に保存されれば完了です。

索引モデルが issue の本文から書いたものです。