I get an error message after my laptop sleeps, before login appears
ACPI errors like that are typically due to your kernel not being a great match to your hardware. Typing "Dell XPS 15 9500 Ubuntu kernel issue" shows many such results, some about suspending issues, suggesting that I'm making a good guess. The Dell XPS 15 9500 is a newer machine than Ubuntu 18.04, so this isn't too surprising. I'd suggest updating your kernel, but that's a riskier move than leaving this error alone. I suspect that the issue will disappear when you upgrade to Ubuntu 20.04.