Hi –
I came across the same issue and the solution appears to be some changes to the instructions in the pluggable widget documentation. Specifically:
In TextBox.tsx, delete the lines below. This is because style and class aren’t defined as Text Inputs.
style={this.props.style}
className={this.props.class}
After adding systemProperty key="Label" all usages of `{this.props.style}` and `{this.props.class}` should be removed because labeled widgets do not have them and the styling is applied to the surrounding FormGroup.