Cannot read property 'filter' of undefined

[evantill]

Context

• Discourse is configured with required login setting (see /admin/site_settings/category/login?filter=login).

• Layouts plugin is installed.

• Layout Category list Widget is installed and enabled on all themes.

• Layouts plugin version 0.2).

• Discourse version 2.6.0.beta2 543e972fec.

Actual

The login page display a blank page and there is an error in the debug console:

_ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:4069 Uncaught TypeError: Cannot read property 'filter' of undefined
    at Object.initialize (e873dec102d20e90fb2c24daae6a56ce65472335.js?__ws=discourse.apelsophiebarat.net:38)
    at Object.s.initialize (_application-2a5d8f98ee67bd11be94df82d7e717b97d025d663f47c3c2bf412ba973929a50.js:6968)
    at _ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:49347
    at i.each (_ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:67724)
    at i.walk (_ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:67638)
    at n.each (_ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:67568)
    at n.topsort (_ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:67576)
    at t._runInitializer (_ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:49361)
    at t.runInstanceInitializers (_ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:49345)
    at t._bootSync (_ember_jquery-d0ba1603b80181031290e4bd850857d41d63bc4a081d7b4bf16d4e1a4e355a22.js:46813)

The error Uncaught TypeError: Cannot read property 'filter' of undefined is on this line :

var categories = site.categories.filter(function (c) {
        return !c.isUncategorizedCategory;
      });

I think that testing site.categories is not null before continuing should fix the problem. I will test it and can make a PR.

Expected behavior

• Login page should work without errors.
• The widget should not be displayed as there are no available categories.