Cannot apply changes in the project settings dialog

[IoeCmcomc]

Describe the bug
Clicking the "OK" button in the project settings dialog do nothing visually, which make it impossible to change the project settings.

To Reproduce
Steps to reproduce the behavior:

1. Open any file
2. From the menu bar, click on Project > Settings
3. (Optional) Change something in the dialog, such as the project name
4. Click the "OK" button
5. Nothing happened

Expected behavior
The dialog should close, and the changes should be saved.

Your Environment:

• OS: Windows 11 23H2
• Python version: 3.8.10
• Pygubu version: 0.35.1
• Designer version: 0.39.2

Additional context
Here's the full console output:

python: 3.8.10 on win32
tk: 8.6
pygubu: 0.35.1
pygubu-designer: 0.39.2
Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Users\PC\AppData\Local\Programs\Python\Python38\lib\tkinter\__init__.py", line 1892, in __call__
    return self.func(*args)
  File "D:\Python\NBSTool\.venv\lib\site-packages\pygubudesigner\services\projectsettings.py", line 203, in btn_apply_clicked
    all_valid, new_settings = self.process_forms()
  File "D:\Python\NBSTool\.venv\lib\site-packages\pygubudesigner\services\projectsettings.py", line 179, in process_forms
    form.submit()
  File "D:\Python\NBSTool\.venv\lib\site-packages\pygubu\forms\form.py", line 99, in submit
    self.submit_init()
  File "D:\Python\NBSTool\.venv\lib\site-packages\pygubu\forms\form.py", line 112, in submit_init
    field.widget.wmark_invalid(False)
  File "D:\Python\NBSTool\.venv\lib\site-packages\pygubu\forms\tkwidget.py", line 75, in wmark_invalid
    self.view_manager.mark_invalid(self, state)
  File "D:\Python\NBSTool\.venv\lib\site-packages\pygubu\forms\ttkwidget.py", line 22, in mark_invalid
    style = style.removeprefix("Error.")
AttributeError: 'str' object has no attribute 'removeprefix'

[alejandroautalan]

Hello @IoeCmcomc, thanks for trying pygubu and for the bug report.
I will try to fix it ASAP.

Regards
Alejandro A.