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

@@ -15,15 +15,16 @@ const App = () => (
   </GlobalContext.Provider>
 );
 
-const appElement = document.querySelector('#mailpoet_form_edit');
+window.addEventListener('DOMContentLoaded', () => {
-
+  const appElement = document.querySelector('#mailpoet_form_edit');
-if (appElement) {
+  if (appElement) {
-  initStore();
+    initStore();
-  initBlocks();
+    initBlocks();
-  ReactDOM.render(
+    ReactDOM.render(
-    <React.StrictMode>
+      <React.StrictMode>
-      <App />
+        <App />
-    </React.StrictMode>,
+      </React.StrictMode>,
-    appElement
+      appElement
-  );
+    );
-}
+  }
+});