<ToolbarGroup>Hello</ToolbarGroup>

Results in TypeError: Cannot read property 'displayName' of undefined

The problem is
https://github.com/callemall/material-ui/blob/5212f0d4718675061ea0d0351735966fc3759e12/src/toolbar/toolbar-group.jsx#L118 . It is lacking check for undefined type.