Dynamic content does not represent relevant data after user interaction

### Description

After interaction with the images in the test link user should see the relevant quantity of items, but instead of specific number NULL is displayed.. See the steps below

### Reproduction Steps

**Steps to repro:**

1. Open Safari browser;
2. Open test link: https://playground.amp.dev/?url=https%3A%2F%2Fpreview.amp.dev%2Fdocumentation%2Fexamples%2Finteractivity-dynamic-content%2Fdynamic_content_after_user-interaction&format=websites
3. Click on images
4. See the text below the images

**Actual Result:** amount of selected fruits is not specified, "null" showed instead

**Expected Result:** number of fruits should be updated according to specific image selected by user

Screenshot:
https://drive.google.com/file/d/183kR7dB6p_Y_czU7X_k4boi2PU33IFf2/view?usp=sharing

Video:
https://photos.app.goo.gl/sM5LmYCQDifwZKt9A

Testing environment:
Safari browser Version 15.5 (17613.2.7.1.8)
Problem not detected on mobile/iphone

### Relevant Logs

_No response_

### Browser(s) Affected

Safari

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Open the linked AMP Playground example for dynamic content after user interaction and reproduce the issue in Safari 15.5 by selecting the images. Trace how the selected fruit quantity is displayed, then verify that the text shows the selected number instead of "null" and that the behavior remains correct on the provided example.

Written by the indexing model from the issue text.