When a call to script.evaluate/callFunction encounters an exception, the logic to extract the text is really weak:

text:
  typeof exception === "object"
    ? exception.toString()
    : String(exception),

https://searchfox.org/mozilla-central/rev/15b656909e77d3048d4652b894f79a8c719b4b86/remote/webdriver-bidi/modules/windowglobal/script.jsm#91-95

First this will fail on null as it is an object but has no toString property. But it would also fail on a random object such as { toString: "not a method" } or any other variant.

For null, the test should be covered by extending our exception_details test cases to cover primitives. For random objects, it might be worth adding a mozilla-specific wdspec test. Or we can extract some of that logic to a dedicated helper and add xpcshell tests.

https://hg.mozilla.org/mozilla-central/rev/2eab01956e47

Post-adding to M4 given that this work is part of it and the project was only on-hold when the patch got landed.

The upstream PR got merged via https://github.com/web-platform-tests/wpt/pull/34945.