The MarionetteCommandsActorProxy might return null if a JSWindowActor query fails for an AbortError, if the command is part of the list of NO_RETRY_METHODS.

This can be confusing, especially when its for a command that in theory can not return null (eg an executeScript which should always return a boolean).

We should at least add a trace log here to make investigations easier. We could also consider bubbling up the initial error instead of swallowing and returning null.

I wonder if in case of Execute Script we should catch the AbortError already in the JSWindowActor parent implementation of executeScript and transform it to a JavascriptError?

Or in case of such a case we should raise an unknown error instead? That would clearly result in an error and state what's wrong.

Adding the trace.log is a conservative approach, but I'm also in favor of throwing here.
Individual commands can then decide what to do upon an AbortError. I imagine the actions-related commands commands are fine with swallowing the error.

Having a trace log entry first for this case would be great. Then we can file bugs for individual commands to let them handle a custom behavior in such a situation. Would you be able to create a quick patch for it? I'm happy to review.

https://hg.mozilla.org/mozilla-central/rev/8b975128d2fc