New user registration gives a Call to a member function get() on null

When the link to the new user registration is clicked some fields appear and a error like 0 Call to a member function get() on null.

https://www.mediumplein.nl/index.php?option=com_djclassifieds&view=registration

I'm using Joomla 3.9.10 and DJ Classifieds 3.7.3.
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