"Edit Cards" Not Possible

[fcvm]

Problem description

While studying a deck, I wanted to edit the image occlusion of a card. So I drawed into the image. Everything went fine but when I pressed the "Edit Cards" button the following error occured.

Checklist

Please replace the space inside the brackets with an x if the following items apply:

• [x] I've restarted Anki to see if it helps
• [x] I've verified that I use the latest version of the add-on by redownloading it from AnkiWeb
• [x] I've verified that I use the latest version of Anki by checking at https://apps.ankiweb.net#download
• [x] I've tried to disable other add-ons to see if there are any interactions present
• [ ] My issue disappears when I hold shift while starting Anki.
• [x] I've checked if anyone else reported this problem before by looking through the issue reports. I also checked to see if there is a section about known issues in the add-on description, documentation, or README.

Information about your Anki set-up

Please open Anki, go to Help → About, click on "Copy Debug Info", and paste the result between the backticks below (if the button does not appear you are using an older version of Anki 2.1 and will need to update first):

    
    Anki 2.1.38 (355e4cd5) Python 3.8.6 Qt 5.14.2 PyQt 5.14.2
    Platform: Windows 10
    Flags: frz=True ao=True sv=1
    Add-ons, last update check: 2021-01-26 09:19:03
    
    
    ===Add-ons (active)===
    (add-on provided name [Add-on folder, installed at, version, is config changed])
    Image Occlusion Enhanced for Anki 21 alpha ['1374772155', 2020-04-28T04:21, 'None', '']
    
    ===IDs of active AnkiWeb add-ons===
    1374772155
    
    ===Add-ons (inactive)===
    (add-on provided name [Add-on folder, installed at, version, is config changed])
    

Error message (if any)

If you've received an error message, please copy and paste it between the backticks below:

Caught exception:
Traceback (most recent call last):
  File "aqt\webview.py", line 515, in handler
  File "C:\Users\...\AppData\Roaming\Anki2\addons21\1374772155\add.py", line 317, in <lambda>
    lambda val, choice=choice: self._onEditNotesButton(choice, val))
  File "C:\Users\...\AppData\Roaming\Anki2\addons21\1374772155\add.py", line 333, in _onEditNotesButton
    r = gen.updateNotes()
  File "C:\Users\...\AppData\Roaming\Anki2\addons21\1374772155\ngen.py", line 109, in updateNotes
    (svg_node, mlayer_node) = self._getMnodesAndSetIds(True)
  File "C:\Users\...\AppData\Roaming\Anki2\addons21\1374772155\ngen.py", line 190, in _getMnodesAndSetIds
    mask_doc = minidom.parseString(self.new_svg.encode('utf-8'))
AttributeError: 'NoneType' object has no attribute 'encode'