Address empty array handling in quiz question settings

[marcellamaki]

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[kolibri-frontend] TypeError: Cannot read properties of null (reading 'reduce')

Slack Message

Trace:

TypeError: Cannot read properties of null (reading 'reduce')
    at addableQuestionCount (webpack://kolibri-root/kolibri/plugins/coach/assets/src/views/quizzes/CreateExamPage/sidePanels/QuizResourceSelection/subPages/QuestionsSettings.vue:94:1)
    at <object>.value (./node_modules/vue/dist/vue.runtime.common.prod.js:11:24951)
    at <object>.value (./node_modules/vue/dist/vue.runtime.common.prod.js:11:25830)
    at get c.value (./node_modules/vue/dist/vue.runtime.common.prod.js:11:21763)
    at <object>.get (./node_modules/vue/dist/vue.runtime.common.prod.js:11:8805)
    at render (./kolibri/plugins/coach/assets/src/views/quizzes/CreateExamPage/sidePanels/QuizResourceSelection/subPages/QuestionsSettings.vue:18:28)
    at t.prototype._render (./node_modules/vue/dist/vue.runtime.common.prod.js:11:44429)
    at o (./node_modules/vue/dist/vue.runtime.common.prod.js:11:74040)
    at <object>.value (./node_modules/vue/dist/vue.runtime.common.prod.js:11:24951)
    at wn (./node_modules/vue/dist/vue.runtime.common.prod.js:11:24890)
    at t (./node_modules/vue/dist/vue.runtime.common.prod.js:11:74054)
    at xo.prototype.$mount (./node_modules/vue/dist/vue.runtime.common.prod.js:11:73929)
    at Zn.init (./node_modules/vue/dist/vue.runtime.common.prod.js:11:29532)
    at n (./node_modules/vue/dist/vue.runtime.common.prod.js:11:33371)
    at <anonymous> (./node_modules/vue/dist/vue.runtime.common.prod.js:11:62186)
    at d (./node_modules/vue/dist/vue.runtime.common.prod.js:11:62073)
    at <anonymous> (./node_modules/vue/dist/vue.runtime.common.prod.js:11:65379)
    at O (./node_modules/vue/dist/vue.runtime.common.prod.js:11:64927)
    at <anonymous> (./node_modules/vue/dist/vue.runtime.common.prod.js:11:65086)
    at O (./node_modules/vue/dist/vue.runtime.common.prod.js:11:64927)
    at <anonymous> (./node_modules/vue/dist/vue.runtime.common.prod.js:11:65086)
    at O (./node_modules/vue/dist/vue.runtime.common.prod.js:11:64927)
    at __patch__ (./node_modules/vue/dist/vue.runtime.common.prod.js:11:66745)
    at t.prototype._update (./node_modules/vue/dist/vue.runtime.common.prod.js:11:43408)
    at o (./node_modules/vue/dist/vue.runtime.common.prod.js:11:74030)
    at <object>.value (./node_modules/vue/dist/vue.runtime.common.prod.js:11:24951)
    at <object>.value (./node_modules/vue/dist/vue.runtime.common.prod.js:11:25594)
    at Vn (./node_modules/vue/dist/vue.runtime.common.prod.js:11:27459)
    at e (./node_modules/vue/dist/vue.runtime.common.prod.js:11:20120)
    at Ye (./node_modules/vue/dist/vue.runtime.common.prod.js:11:19552)

[sentry]

Sentry Issue: KOLIBRI-FRONTEND-13XK