Clicking 'Start a new discussion...' fails to expand interface

For !36347 (merged), when clicking on 'Start a new discussion...' where a previous, resolved discussion exists, it fails to expand the interface and throws JS errors in the console:

TypeError: undefined is not an object (evaluating 'r.highlighted_diff_lines.find((function(e){return e.line_code===i})).hasForm=s')

TypeError: r.highlighted_diff_lines.find(...) is undefined
    TOGGLE_LINE_HAS_FORM mutations.js:105
    m vuex.esm.js:840
    commit vuex.esm.js:462
    commit vuex.esm.js:461
    _withCommit vuex.esm.js:620
    commit vuex.esm.js:460
    commit vuex.esm.js:405
    commit vuex.esm.js:792
    Le mutation_types.js:13
instrument.js:108:45
    pt instrument.js:108
    VueJS 7
    onClick diff_discussion_reply.vue:1
    VueJS 4
    click discussion_reply_placeholder.vue:1
    VueJS 3
    r helpers.js:72
    (Async: EventListener.handleEvent)
    _wrapEventTarget trycatch.js:137
    pt instrument.js:345
    VueJS 47
    SET_LINE_DISCUSSIONS_FOR_FILE mutations.js:226
    SET_LINE_DISCUSSIONS_FOR_FILE mutations.js:225
    SET_LINE_DISCUSSIONS_FOR_FILE mutations.js:222
    m vuex.esm.js:840
    commit vuex.esm.js:462
    commit vuex.esm.js:461
    _withCommit vuex.esm.js:620
    commit vuex.esm.js:460
    commit vuex.esm.js:405
    commit vuex.esm.js:792
    ke mutation_types.js:18
    ke actions.js:202
    m vuex.esm.js:847
    dispatch vuex.esm.js:512
    dispatch vuex.esm.js:402
    dispatch vuex.esm.js:775
    r vuex.esm.js:1060
    setDiscussions app.vue:378
    (Async: requestIdleCallback handler)
    setDiscussions app.vue:376
    unwatchDiscussions app.vue:288
    VueJS 10
    SET_DIFF_DATA_BATCH mutations.js:73
    m vuex.esm.js:840
    commit vuex.esm.js:462
    commit vuex.esm.js:461
    _withCommit vuex.esm.js:620
    commit vuex.esm.js:460
    commit vuex.esm.js:405
    commit vuex.esm.js:792
    e mutation_types.js:7
    (Async: promise callback)
    e actions.js:82
    Se actions.js:127
    m vuex.esm.js:847
    dispatch vuex.esm.js:512
    dispatch vuex.esm.js:402
    dispatch vuex.esm.js:775
    r vuex.esm.js:1060