unfortunately I can test this in dev...

static/admin/prefixed-image-widget.js

@@ -1,4 +1,11 @@
-class PrefixedImageControl extends React.Component {
+(function waitForCMSAndReact() {
+  if (!(window.CMS && window.React)) {
+    setTimeout(waitForCMSAndReact, 50);
+    return;
+  }
+  const React = window.React;
+
+  class PrefixedImageControl extends React.Component {
     openMediaLibrary = () => {
       this.props.mediaLibrary.open({
         allowMultiple: false,
@@ -31,17 +38,18 @@ class PrefixedImageControl extends React.Component {
         )
       );
     }
-}
+  }
 
-const PrefixedImageWidget = {
+  const PrefixedImageWidget = {
     name: "prefixed-image",
     controlComponent: PrefixedImageControl,
     previewComponent: ({ value }) =>
       value ? React.createElement("img", { src: value, alt: "", style: { maxWidth: "200px" } }) : null,
-};
+  };
 
-window.CMS.registerWidget(
+  window.CMS.registerWidget(
     PrefixedImageWidget.name,
     PrefixedImageWidget.controlComponent,
     PrefixedImageWidget.previewComponent
-);
+  );
+})();