Issue with ra-ui-materialui?

What you were expecting:

Yesterday everything was working fine and I’m wondering if this is because of an update on your side. I was expecting to get access to my interface.

What happened instead:

Got this issue in ra-ui-materialui and I don’t know why:

TypeError: Cannot read property 'breakpoints' of null
(anonymous function)
node_modules/ra-ui-materialui/esm/layout/AppBar.js:71
  68 | var children = _a.children, classesOverride = _a.classes, className = _a.className, logo = _a.logo, logout = _a.logout, open = _a.open, title = _a.title, userMenu = _a.userMenu, rest = __rest(_a, ["children", "classes", "className", "logo", "logout", "open", "title", "userMenu"]);
  69 | var classes = useStyles({ classes: classesOverride });
  70 | var dispatch = useDispatch();
> 71 | var isXSmall = useMediaQuery(function (theme) { return theme.breakpoints.down('xs'); });
     | ^  72 | return (React.createElement(HideOnScroll, null,
  73 |     React.createElement(MuiAppBar, __assign({ className: className, color: "secondary" }, rest),
  74 |         React.createElement(Toolbar, { disableGutters: true, variant: isXSmall ? 'regular' : 'dense', className: classes.toolbar },

Steps to reproduce:

Followed the demo, and started adding a few things. Everything was working fine until today. I am not 100% sure if this is on my side or not, so sorry in advance if not.

Related code:

Let me know if I can join anything.

• Preferably, a CodeSandbox forked from https://codesandbox.io/s/github/marmelab/react-admin/tree/master/examples/simple
• A link to a GitHub repo with the minimal codebase to reproduce the issue

insert short code snippets here

Other information:

Sorry in advance if my request is not good enough, I’m still a beginner. Please, let me know.

Environment

• React-admin version: 3.0.3 (but having same issue with 3.1.3)
• Last version that did not exhibit the issue (if applicable):
• React version: 16.12.0
• Browser: Chrome
• Stack trace (in case of a JS error):

[fzaninotto]

What’s the version of material-ui that you’ve installed? Can you check in your packages.lock that there is only one version installed?

[iNukeLaPeste]

Needed to add @material-ui/core but it didn’t work when I first tried. Now it works, thank you! "@material-ui/core": "^4.5.1"

Sorry for the issue.