AppBar overlaps

import withStyles from '@material-ui/core/styles/withStyles';
const styles = theme => ({
  appBarSpacer: theme.mixins.toolbar
});

const style = withStyles(styles)

function MyScreen ({ classes }) {
  <AppBar></AppBar>
    <div className={classes.appBarSpacer}></div>
  <Box></Box>
}

export default style(MyScreen)