Each child in a list should have a unique "key" prop. Check the render method of `InsertModalHeader`

Receiving the error:

Each child in a list should have a unique "key" prop. Check the render method of `InsertModalHeader`

The reason is that the line:
return [ title, closeBtn ];
renders 2 children, the title and the closeBtn. The title has a "key" property. But, the closeBtn does not have a "key" property.

This only happens in bootstrap 4 mode where these two components are rendered as an array.

Solution #1: Add a "key" property to closeBtn

Solution #2: Render each element but not in an array.