Initialize form editor after dom is loaded

We need window.wpApiSettings which is initialized by WordPress in page footer.
[MAILPOET-2750]

assets/js/src/form_editor/form_editor.jsx

@@ -15,9 +15,9 @@ const App = () => (
   </GlobalContext.Provider>
 );
 
-const appElement = document.querySelector('#mailpoet_form_edit');
-
-if (appElement) {
+window.addEventListener('DOMContentLoaded', () => {
+  const appElement = document.querySelector('#mailpoet_form_edit');
+  if (appElement) {
     initStore();
     initBlocks();
     ReactDOM.render(
@@ -26,4 +26,5 @@ if (appElement) {
       </React.StrictMode>,
       appElement
     );
-}
+  }
+});