Messenger creates a session for storing flash messages even when response format isn't HTML/it will never show

I actually went ahead and applied this changed to one of the projects I'm maintaining and wanted to report a regression I just noticed.
We're using the VBO export module to generate CSV exports, and the download link is no longer output because of this change, guessing because the BATCH API is using Ajax.

Additionally, note that the following:

$this->requestStack->getCurrentRequest() can potentially return NULL.

Unfortunately, while this change is appealing, I don't think it can be as simple as the fix suggested in the MR.

$this->requestStack->getCurrentRequest() can potentially return NULL.

Unfortunately, while this change is appealing, I don't think it can be as simple as the fix suggested in the MR.

Thanks Jonathan for taking a look.

I think the null-safety for the current request is easy enough to fix - we could just no-op and keep the current behavior.

As for AJAX, I had forgotten about that (pretty much all my traffic is decoupled) but it is very much a thing - I think the test could be request format html OR ajax? That would keep this pretty tight?

Updated this to also consult the "wrapper format" for the no-op to address #5.