Here https://github.com/moodle/moodle/blob/master/mod/assign/amd/src/participant_selector.js at row 111 is defined ).catch(
But it throws JS error - "Uncaught TypeError: a.call(...).then(...).then(...).catch is not a function"
Checked the overall Moodle code and catch is not actually used anywhere. Instead ).fail( is used in many places. I replaced catch with fail and JS error is gone.
Just a quick note. Hope this helps.