Mixin working in template but not from computed property

In vue.js component I'm using mixins from supply store declared like this:

mixins: [
    use('Categories'),
    ],

    computed: {
        ...mapGetters({
            getMainCategories: 'categories/getMainCategories',
            ss: 'categories/getSubcategories',
        }),
(...)

It is working fine when I access it directly in template {{getMainCategories}}, but when I try to access it from another computed property:

 categoriesSelectValues() {
        if (this.getMainCategories.length > 0) {

            }
            else return []
        }

I get

[Vue warn]: Error in render: "TypeError: this.getMainCategories is undefined"

I completely don't know what's wrong, especially that I have another - almost the same component - which is working just fine.