Inefficient, yes. But no input validation is not a knock against this solution.

As per the details of this Kata:
"All the test strings would contain valid Morse code, so you may skip checking for errors and exceptions."

Extremely unefficient solution with so many nested loops and totally unnecessary toList conversion.
Also, to me it doesn't look like so expressive.
Last but not least, there is no input validation at all.

Far away to be considered as 'Best practice'.