When I start Kate as "kate /dev/stdin", I get the message: The file /dev/null could not be loaded, as it was not possible to read from it. Check if you have read access to this file. Why /dev/null? I provided /dev/stdin as the argument.
Kate/KWrite can't work with such device files. Why it doesn't report /dev/stdin? That depends to which device stdin is connected, in your case dev/null, as perhaps stdin was closed, I get here some pts/X device for my virtual console.