HitBox (Kong version): Freeze after completing level

[n0samu]

Describe the bug

In the Kongregate version of HitBox, the game freezes after completing a level. This only happens when the game is run directly from Kongregate. The problem seems to be caused by Kongregate's online API SWF (and not by its local "shadow" API SWF). To reproduce the problem with the Ruffle extension, you can open this URL: https://chat.kongregate.com/gamez/0026/3025/live/hitBox_kong_730x550.swf?kongregate_api_path=https%3A%2F%2Fchat.kongregate.com%2Fflash%2FAPI_AS3_d43c4b859e74432475c1627346078677.swf

This error is repeated in the console:
ERROR core/src/avm2/activation.rs:930 TypeError: Error #1009: Cannot access a property or method of a null object reference. (accessing field: ::submit)

Expected behavior

It should be possible to continue the game after beating a level.

Affected platform

Self-hosted version

Operating system

Windows 10

Browser

Firefox 109

Additional information

It's possible to reproduce the same problem in Flash Player by giving the game an incorrect API URL, for example:
https://chat.kongregate.com/gamez/0026/3025/live/hitBox_kong_730x550.swf?kongregate_api_path=https%3A%2F%2Fchat.kongregate.com%2Fflash%2Finvalid.swf

I also noticed that on the Kongregate page, Ruffle is loading the wrong URL for the API SWF:
https://game263025.konggames.com/games/gameshot/hitbox/frame/7fe8db98-d738-48df-bdc4-f531068023ac/https%3A%2F%2Fchat.kongregate.com%2Fflash%2FAPI_AS3_d43c4b859e74432475c1627346078677.swf
I have not been able to figure out why the game loads the wrong URL when run from the Kongregate page. But I found that the game loads the correct URL when run directly from the SWF URL provided in the "describe the bug" section above.

Even when the game successfully loads the API SWF, in Ruffle the problem still occurs.

[n0samu]

Now the problem only occurs on the Kongregate game page (https://www.kongregate.com/games/gameshot/hitbox), not on the extension page or in the desktop app. That means it only occurs when the Kongregate API SWF fails to load. And the API SWF fails to load on the Kongregate game page because Ruffle tries to load it via the wrong URL, for some reason. (See the "Additional Information" section above.)

[n0samu]

The problem occurs because Kongregate is passing URL-encoded swfobject_flash_vars directly to Ruffle's parameters variable, when Ruffle takes non-URL-encoded variables. This is a problem with Kongregate's setup, not with Ruffle.