Fix drag-and-drop in strict mode (issue #542)

Author: raymond-lam

example/package-lock.json

@@ -4,7 +4,11 @@ import PlaygroundContainer from '../components/PlaygroundContainer';
 import 'bootstrap/dist/css/bootstrap.min.css';
 
 const App = () => {
-  return <PlaygroundContainer title='React JSON Schema Form Builder' />;
+  return (
+    <React.StrictMode>
+      <PlaygroundContainer title='React JSON Schema Form Builder' />
+    </React.StrictMode>
+  );
 };
 
 export default App;