fix: 'NoneType' object has no attribute 'replace'
    
    Caused by changes how way 'StringField' is handling the default value in 'wtforms' >= 3.0.0.
    In previous versions, 'None' was changed to an empty string, now it stays 'None'.
    Logic added to set the affected variable to 'str()'.
    
    References:
    https://github.com/wtforms/wtforms/commit/29c60458f3809bb43fb4007cfe710f3d7f4067d4
    https://github.com/wtforms/wtforms/issues/291