The prop `children` is marked as required in `Button`, but its value is `undefined`
"children" are what is between the tags:
<Button>we are the children</Button>
Because you don't pass anything - it's undefined. To fix error just add something that React can render, like string:
<Button
variant="raised"
color="primary"
className={classes.Button}
label={this.state.buttonLabel}
onClick={this.handleClick}
>
My Button
</Button>;
Looks like they removed the label
property and moved it to the children
prop.
So you have to put it between the tags e.g.
<Button> here comes the label </Button>
You can always check the demos for the Components. See here