Searching in built-in search dialogs can crash Picard.

Underlying reason seems to be that the center table can get deleted to show the spinner, but events might still trigger trying to access it. E.g.:

Traceback (most recent call last):
File "./picard/webservice/_init_.py", line 442, in _process_reply
self._handle_reply(reply, request)
File "./picard/webservice/_init_.py", line 429, in _handle_reply
handler(document, reply, error)
File "./picard/ui/searchdialog/album.py", line 223, in handle_reply
self.display_results()
File "./picard/ui/searchdialog/album.py", line 340, in display_results
self.prepare_table()
File "./picard/ui/searchdialog/_init_.py", line 349, in prepare_table
self.table.itemSelectionChanged.connect(
AttributeError: 'NoneType' object has no attribute 'itemSelectionChanged'