Dialogs in Edit Book cause unhandled exception

[Corwin Watts]

I was attempting to edit a epub after updating to the new Calibre 3.45 and any attempt to use an edit function that requires a dialog such as Manage Fonts, Compress Images Losslessly, and Remove Unused Css Rules caused an unhandled exception. In addition, Manage Fonts and Compress Images cause a loop that prevents them from being closed and prevents saving of the epub. It can also cause an exception dialog on closing if I don't use Manage Fonts, Compress or Remove, but that one goes away after I click OK. In all cases the same error dialog is reported and sent to the clipboard. I have included it below:

calibre, version 3.45.0
ERROR: Unhandled exception: <b>SystemError</b>:error return without exception set

calibre 3.45 [64bit] embedded-python: True is64bit: True
Windows-7-6.1.7601-SP1 Windows ('64bit', 'WindowsPE')
('Windows', '7', '6.1.7601')
Python 2.7.15+
Windows: ('7', '6.1.7601', 'SP1', u'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: DeDRM (6, 5, 3) && Resize Cover (1, 0, 2) && Goodreads (1, 4, 0) && Open With (1, 5, 10) && Generate Cover (1, 5, 21) && Wiki Reader (2, 1, 0) && Modify ePub (1, 3, 13) && Find Duplicates (1, 6, 3) && EpubMerge (2, 5, 0) && Quality Check (1, 9, 11) && Barnes & Noble (1, 2, 15)
Traceback (most recent call last):
  File "site-packages\calibre\gui2\widgets2.py", line 188, in reject
  File "site-packages\calibre\utils\config.py", line 393, in set
  File "site-packages\calibre\utils\config.py", line 448, in __setitem__
  File "site-packages\calibre\utils\config.py", line 411, in commit
  File "site-packages\calibre\utils\config.py", line 432, in to_raw
  File "site-packages\calibre\utils\config_base.py", line 86, in json_dumps
  File "json\__init__.py", line 251, in dumps
  File "json\encoder.py", line 209, in encode
  File "json\encoder.py", line 434, in _iterencode
  File "json\encoder.py", line 408, in _iterencode_dict
  File "json\encoder.py", line 313, in _iterencode_list
  File "json\encoder.py", line 39, in encode_basestring
SystemError: error return without exception set

In addition the calibre parallel threads and their conhost.exe do not close down but remain in memory after I exit calibre.

[Kovid Goyal (kovid)]

There is a windows specific issue in calibre 3.45.0 causing this error. I am
in the process of tracking it down, when I do I will make a new calibre release.
In the meantime I suggest downgrading to 3.44.0, which you can get from here:
https://download.calibre-ebook.com/3.44.0/

 status fixreleased

[Kovid Goyal (kovid)]

There is a windows specific issue in calibre 3.45.0 causing this error. I
have released version 3.45.1 with the fix. Update and you will be fine.

 status fixreleased