Mashup no subcategories visible

I just bought the Joomla 4 version. And I used it to show the subcategories also. It worked ok in Joomla 3.
But when I use this newer version and activate the subcategories now with (-Child Category Articles: Include, - Category Depth: 3) I get an error:

#0 ARRAY_MERGE(): ARGUMENT #1 MUST BE OF TYPE ARRAY, STRING GIVEN
D:WAMPWWWYOGASTUDIEMODULESMOD_JUX_MASHUPHELPER.PHP:107

107: $catids = array_unique(array_merge($catids, $additional_catids));

The website is running local in WAMP.

Can you provide a solution ?

Best Regards,
Philip Raaijmakers