The website currently doesn't allow using a quote or apostrophe in a joker name (such as "Joker's"). However, this is possible within the engine by escaping the character with a backslash. So for example, "Joker\'s".

So ' should either be automatically replaced by \' within the saved data, or at least the current warning message "Name cannot contain quotation marks" should be replaced by something more appropriate without blocking the save function. For example, "Warning: One or more quotation marks were detected in the joker's name. Remember to escape them using a backslash like \' otherwise your mod might crash."