Describe the bug

I've just assigned a skill to a subtopic. When I try to select the same skill for the topic's diagnostic test, I expect the diagnostic test skill to remain after I've saved my changes to the topic. Instead, the diagnostic test skill disappears upon saving my changes, as if I never added the diagnostic test skill in the first place.

URL of the page where the issue is observed.

/topic_editor/<topic-id>#

Steps To Reproduce

1. Go to the Topic and Skills Dashboard as a Curriculum Admin.
2. Create a new skill. Create 3 questions for the skill.
3. Create a new topic.
4. Assign the newly-created skill to the topic.
5. Edit the topic. Create a subtopic under the topic. Assign the skill to the subtopic.
6. Try adding the skill as a diagnostic test skill for the topic. Save the changes when you think you've selected the skill for diagnostic test.
7. Observe the diagnostic test skill section. Notice that the skill you thought you selected disappears as if you never added the diagnostic test skill in the first place.

Expected Behavior

I expect to be able to add a test diagnostic skill along with adding the same skill to a subtopic, while being able to save both those changes simultaneously. Having this expectation met makes it more efficient to meet the requirements for publishing a topic.

While reloading the page fixes this behavior (as seen in the video), the app shouldn't expect the user to this, especially without explicit warning.

Screenshots/Videos

What device are you using?

Desktop

Operating System

Linux

What browsers are you seeing the problem on?

Chrome

Browser version

121.0.6167.85

Additional context

Assigning the skill to the topic and choosing the same skill for diagnostic test must be done on the same save without switching pages. Otherwise, the bug won't occur.

Tips for developers

Before addressing the bug, please identify which PR caused the issue (you can follow the steps here). If you identify the PR, comment on the issue with a link to it. If not, mention the commit hash of the oldest commit you saw the bug on (and the month and year it was made in).

Also, if this is your first issue, please make sure to follow https://github.com/oppia/oppia/wiki/Contributing-code-to-Oppia#choosing-a-good-first-issue and https://github.com/oppia/oppia/wiki/Contributing-code-to-Oppia#setting-things-up before claiming it. Thanks!