Crash when using `mode: library` with number of files != 1

[forivall]

I'm trying out TypeStrong/typedoc#1184 , and alas, it causes this plugin to crash

node_modules/typedoc-plugin-toc-group/plugin.js:122
                        page.toc.children.forEach((item) => {
                                          ^

TypeError: Cannot read property 'forEach' of undefined
    at TocGroupPlugin.buildGroupTocContent (node_modules/typedoc-plugin-toc-group/plugin.js:122:43)
    at TocGroupPlugin.onBeginRendererPage (node_modules/typedoc-plugin-toc-group/plugin.js:113:18)
    at triggerEvents (node_modules/typedoc/dist/lib/utils/events.js:128:43)
    at triggerApi (node_modules/typedoc/dist/lib/utils/events.js:110:13)
    at eventsApi (node_modules/typedoc/dist/lib/utils/events.js:21:18)
    at Renderer.trigger (node_modules/typedoc/dist/lib/utils/events.js:264:13)
    at Renderer.renderDocument (node_modules/typedoc/dist/lib/output/renderer.js:43:14)
    at node_modules/typedoc/dist/lib/output/renderer.js:36:22
    at Array.forEach (<anonymous>)
    at Renderer.render (node_modules/typedoc/dist/lib/output/renderer.js:35:25)