Only assign opener when not using nativeWindowOpen

lib/renderer/window-setup.js

@@ -127,6 +127,10 @@ module.exports = (ipcRenderer, guestInstanceId, openerId, hiddenPage, usesNative
         return null
       }
     }
+
+    if (openerId != null) {
+      window.opener = getOrCreateProxy(ipcRenderer, openerId)
+    }
   }
 
   window.alert = function (message, title) {
@@ -142,10 +146,6 @@ module.exports = (ipcRenderer, guestInstanceId, openerId, hiddenPage, usesNative
     throw new Error('prompt() is and will not be supported.')
   }
 
-  if (openerId != null) {
-    window.opener = getOrCreateProxy(ipcRenderer, openerId)
-  }
-
   ipcRenderer.on('ELECTRON_GUEST_WINDOW_POSTMESSAGE', function (event, sourceId, message, sourceOrigin) {
     // Manually dispatch event instead of using postMessage because we also need to
     // set event.source.