Using kint in views-view-unformatted
Option 1: Try to increase your php memory limit above the 250 MB.

Option 2: Don't try to debug all the rows at once.

{{ for row in rows }}
  {{ kint(row) }}
{{ endfor }}


P.S.

The reason {{ row }} was null, was because that variable does not exist in the template unless declared like in my example above.