React child Component in parent Component

Solution 1:

The problem is quite clear: The Button component does not return anything when the conditions do not match.

You just need to add a simple return null at the end of the render method for Button:

render() {
    const btn = this.props;

    ...

    return null;
    
}

Another option is to add one of the missing props to the Button when you use it:

<Button theme="primary" type="button">Contact us</Button>

<Button theme="primary" type="link">Contact us</Button>

In either case, I recommend you keep the return null inside the render method.