Fix filter dialog crashing on Ctrl+Space

KrIr17requested to merge
KrIr17/inkscape:krir_filter_crash into 1.4.x

After Ctrl+Space clears selection, get_selected() returns an invalid iterator, comparison of which causes a crash. Fix by checking if result of get_selected() is valid.

1.4.x counterpart of !7177 (merged)

Fixes #5569 (closed)

Edited by KrIr17

Merge request reports