When creating a new archive from an Ark instance where an archive with comment already is opened, the comment widget from the previous archive remains open. This is now fixed by clearing and hiding the QPlaintTextEdit in start of Part::openFile().

Part::slotCommentChanged() gets called when calling QPlainTextEdit::clear(), so to avoid a segfault we need to check for a invalid m_model.archive() and in that case return.

1. Open any archive with a comment.
2. Create a new archive from Archive menu.
3. Verify that the comment field from the previous archive disappears.