Can't select viewmode

Hi @UriDrupal, could you verify first if your issue is similar to:
https://www.drupal.org/project/paragraph_view_mode/issues/3157506
especially if you could verify if you enabled those view modes on your paragraph type.

@UriDrupal I would be very glad if you can answer my question or provide me with any additional information to debug your issue.

The error has been fixed. I added the patch.

Thank you for introducing the patch. I maybe not follow, but could you explain why the __CLASS__ is better then get_called_class()? The second one is more flexible, as it allows to call the method from the current class - so it's usefull in case of inheritance. I would rather stick to get_called_class, but I would like to know opinion from the community.
I also don't understand why did you change the method to a static one? Even in Ajax Form API examples a non-static methods are used.

why the __CLASS__ is better then get_called_class()

It's my personal opinion. I'm used to see php callable array with this constant. Maybe it's not a very good style in drupal context.

I also don't understand why did you change the method to a static one?

It was the reason of the error. How are you going to use a non-static method in php callable? In this case you have to set an object as the first element of callable-array. But as you already said, get_called_class() returns the classname string, not object, __CLASS__ is the same.

Even in Ajax Form API examples a non-static methods are used.

It's just a callback. You can use any callable. Read more about ajax callbacks

Ok I see your point. IMO get_called_class() could better because it's like with a static vs self so basicaly get_called_class will return the current executed class, the __CLASS__ will return the specific class - ParagraphViewModeWidget.
Of course, it can be altered - it's a matter of changing the callback in the form element, but I think that the current solution is a bit more clean solution. What is your opinion on this?

In terms of changing a method into static, yes I totally get it now, so it's definitely a great thing :-)