About/Credits not popping up

[KilianMaes]

Describe the bug:
Hello,
When I run OpenShot (pulled from the "develop" branch), and I want to access the credits through Help > About OpenShot > Credits, the Credits window doesn't pop up and an error message is printed in the terminal.

Steps to reproduce the behavior:

1. Go to Help > About OpenShot
2. Click on Credits

Expected behavior:
A window containing the Credits pops out.

System Details:

• OpenShot Version: 2.5.1-dev2
• Operating System / Distro: Ubuntu 20.04.1 LTS
• Python: 3.8.5

Log Files:
openshot-qt.log

Exception / Stacktrace:
Traceback (most recent call last):
File "/home/maesk/openshot/openshot-qt/src/windows/about.py", line 195, in load_credit
windo = Credits()
File "/home/maesk/openshot/openshot-qt/src/windows/about.py", line 287, in init
self.developersListView = CreditsTreeView(
File "/home/maesk/openshot/openshot-qt/src/windows/views/credits_treeview.py", line 88, in init
self.refresh_view()
File "/home/maesk/openshot/openshot-qt/src/windows/views/credits_treeview.py", line 47, in refresh_view
self.credits_model.update_model(filter=filter)
File "/home/maesk/openshot/openshot-qt/src/windows/models/credits_model.py", line 59, in update_model
data = defaultdict("")
TypeError: first argument must be callable or None

Solving the bug
I wanted to confirm that other people (maybe on other platforms) have the same bug, to be sure that the problem doesn't come from my configuration. I would like to solve this bug, I'm going to propose a PR very soon.