I can't think of an easy workaround that wouldn't require us to ship a fix – but we're about to publish a patch version anyways to fix a few other small issues, so we'll definitely include this.
Edit: Actually, an easy workaround would be to add a hack to the give_answers endpoint that filters out empty tasks received back from the client before they're passed back to the controller.
Also, I'm pretty sure that something must have changed in the latest version (or one of the latest versions) that allowed the client to produce "empty" tasks on rapid clicks. The issue just came up again here and it's never come up before. So it's unlikely to be a long-standing issue and likely a regression introduced in v1.8.4 (which should also help narrow it down).
Edit 2: Found and fixed the issue. It was indeed a regression introduced only in v1.8.4.
Edit 3: Just released v1.8.5, which should resolve the issue! Thanks again for the report 