The easiest solution is to first create a variable that you will place in your return statement.
What I mean is to create a variable which will hold your current JSX.
Then you can decide if the stuff you return contains the extra div or not.
Very simple example:
let mainDiv = (<div>my stuff </div>);
let extraDiv = (<div> extra stuff</div>);
let targetDivsForReturn = [];
render(){
targetDivsForReturn.push(mainDiv);
if (isShown){
targetDivsForReturn.push(extraDiv);
}
return (<>
{targetDivsForReturn}
</>)
}