This appears to be reproducible for me. Attached are the afdesign file and two back-to-back crash reports.
Context:
I was taking an existing file as the base for a new file and scrubbing through history (using the slider) to get back near the beginning of the process so I could shortcut to reuse of assets and some of the previous work. The twitchy nature of using that slider means that I probably went back and forth a couple of times; I haven't identified a specific cheat-code that triggers the crash but it didn't take much movement.
EDIT: Apparently I don't have to do much back and forth scrubbing at all. I just moved the slider from latest to earliest without any intentional hesitation and *BOOM*. ("There's always a boom." - Ivanova)
Environment: macOS Mojave 10.14.6, MBP2018, 32GB, Radeon Vega Pro 20, 2.9GHz i9, external display: Cinema Display HD 30 through OWC Dock/Mini DisplayPort to DualLink DVI Apple adapter.
Affinity Designer_2019-08-25-145121_SpamBook2018.crash
Affinity Designer_2019-08-25-145416_SpamBook2018.crash
fading_daylight_2019_1.afdesign