"This page crashed. can't convert Object to string" in playground

**Describe the bug**
A clear and concise description of what the bug is.

**To Reproduce**
Please provide a GitHub link or code snippet.

https://ffmpegwasm.netlify.app/playground/

Under "Edit arguments below to update command", add {"toString": "1"} as an argument:
```json
[
"-i",
"video.webm",
"video.mp4",
{"toString": "1"}
]
```

Result (immediately occurs after typing the } in {"toString": "1"}):

"This page crashed. can't convert Object to string"

**Expected behavior**
A clear and concise description of what you expected to happen.

No error message.

**Screenshots**
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**Desktop (please complete the following information):**
Firefox 128 on Debian Linux

**Smartphone (please complete the following information):**
- Device: [e.g. iPhone6]
- OS: [e.g. iOS8.1]
- Browser [e.g. stock browser, safari]
- Version [e.g. 22]

**Additional context**
Add any other context about the problem here.

Code should verify that the top level JSON object is an array and that each item of the array is a string.