[BUG]: 404 Not Found error on valid collection player page.

[jnvtnguyen]

Describe the bug

When I visit a valid collection player page, I get a 404 Not Found error.

URL of the page where the issue is observed.

/collection/

Steps To Reproduce

1. Create a new collection (need collection editor rights).
2. Create/add a exploration to the collection and publish it.
3. Go to the collection player of that collection.

Expected Behavior

I expect there to be no 404 Not Found error.

Screenshots/Videos

What device are you using?

Desktop

Operating System

Windows

What browsers are you seeing the problem on?

Chrome

Browser version

No response

Additional context

The other parameter "key" error is a different issue which I filed here: #20198.

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