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

@@ -15,8 +15,8 @@ const App = () => (
   </GlobalContext.Provider>
 );
 
+window.addEventListener('DOMContentLoaded', () => {
   const appElement = document.querySelector('#mailpoet_form_edit');
-
   if (appElement) {
     initStore();
     initBlocks();
@@ -27,3 +27,4 @@ if (appElement) {
       appElement
     );
   }
+});