Summary
When trying to access https://pre-release.mattermost.com it crash to a blank screen.
Steps to reproduce
Using Firefox 57.0.4 or Chromium 62.0.3202.89 on Linux (Debian Buster), try access to https://pre-release.mattermost.com. After login page, the Web UI load and crash suddenly.
Expected behavior
It should just works fine 
Observed behavior
When looking at my Web console, it show that the browser try to make a POST request to https://pre-release.mattermost.com/api/v4/roles/names which doesn’t seems to exists and returned a 404 error. Then a React error Cannot read property 'top' of undefined pop and UI stopped to load. I just dumped you the error message, not sure it help
Error log
TypeError: Cannot read property 'top' of undefined
at t.updateUnreadIndicators (sidebar.jsx:308)
at t.value (sidebar.jsx:149)
at updateClassInstance (react-dom.production.min.js:115)
at beginWork (react-dom.production.min.js:137)
at r (react-dom.production.min.js:164)
at i (react-dom.production.min.js:164)
at s (react-dom.production.min.js:165)
at y (react-dom.production.min.js:171)
at v (react-dom.production.min.js:171)
at m (react-dom.production.min.js:169)
l @ react-dom.production.min.js:167
s @ react-dom.production.min.js:165
y @ react-dom.production.min.js:171
v @ react-dom.production.min.js:171
m @ react-dom.production.min.js:169
p @ react-dom.production.min.js:168
enqueueSetState @ react-dom.production.min.js:111
r.setState @ react.production.min.js:15
u.onStateChange @ connectAdvanced.js:209
notify @ Subscription.js:30
e.notifyNestedSubs @ Subscription.js:69
u.onStateChange @ connectAdvanced.js:206
notify @ Subscription.js:30
e.notifyNestedSubs @ Subscription.js:69
u.onStateChange @ connectAdvanced.js:206
u @ createStore.js:176
(anonymous) @ index.js:18
(anonymous) @ middleware.js:85
(anonymous) @ channels.js:1388
a @ runtime.js:69
(anonymous) @ runtime.js:307
e.(anonymous function) @ runtime.js:121
a @ channels.js:66
(anonymous) @ channels.js:66
Promise resolved (async)
a @ channels.js:66
(anonymous) @ channels.js:66
(anonymous) @ channels.js:66
(anonymous) @ channels.js:1401
(anonymous) @ global_actions.jsx:56
Promise resolved (async)
t @ global_actions.jsx:54
o @ global_actions.jsx:93
(anonymous) @ route_team.jsx:59
Promise resolved (async)
i @ route_team.jsx:50
o @ route_team.jsx:33
o @ TransitionUtils.js:47
(anonymous) @ TransitionUtils.js:118
(anonymous) @ TransitionUtils.js:88
r @ AsyncUtils.js:38
a @ AsyncUtils.js:55
a @ TransitionUtils.js:87
r @ TransitionUtils.js:111
(anonymous) @ createTransitionManager.js:74
a @ TransitionUtils.js:78
s @ TransitionUtils.js:135
f @ createTransitionManager.js:71
(anonymous) @ createTransitionManager.js:49
r @ AsyncUtils.js:45
a @ AsyncUtils.js:55
l @ matchRoutes.js:235
r @ createTransitionManager.js:45
n @ createTransitionManager.js:226
b @ createTransitionManager.js:250
componentWillMount @ Router.js:100
mountClassInstance @ react-dom.production.min.js:113
beginWork @ react-dom.production.min.js:137
r @ react-dom.production.min.js:164
i @ react-dom.production.min.js:164
s @ react-dom.production.min.js:165
y @ react-dom.production.min.js:171
v @ react-dom.production.min.js:171
m @ react-dom.production.min.js:169
p @ react-dom.production.min.js:168
updateContainer @ react-dom.production.min.js:177
(anonymous) @ react-dom.production.min.js:227
unbatchedUpdates @ react-dom.production.min.js:174
hn @ react-dom.production.min.js:227
render @ react-dom.production.min.js:229
o @ root.jsx:121
(anonymous) @ root.jsx:98
l @ jquery.js:3321
fireWith @ jquery.js:3451
c @ jquery.js:3646
u @ jquery.js:3655
setTimeout (async)
(anonymous) @ jquery.js:3693
l @ jquery.js:3321
fireWith @ jquery.js:3451
fire @ jquery.js:3459
l @ jquery.js:3321
fireWith @ jquery.js:3451
o.(anonymous function) @ jquery.js:3782
l @ jquery.js:3321
fireWith @ jquery.js:3451
o.(anonymous function) @ jquery.js:3782
(anonymous) @ root.jsx:66
Promise resolved (async)
(anonymous) @ user_actions.jsx:67
a @ runtime.js:69
(anonymous) @ runtime.js:303
e.(anonymous function) @ runtime.js:121
a @ asyncToGenerator.js:21
(anonymous) @ asyncToGenerator.js:32
Promise resolved (async)
a @ asyncToGenerator.js:31
(anonymous) @ asyncToGenerator.js:39
t @ _export.js:39
(anonymous) @ asyncToGenerator.js:18
(anonymous) @ main.6a552575d371e3a8b032.js:26302
r @ root.jsx:66
(anonymous) @ root.jsx:157
window.onload @ root.jsx:147