I have the same issue! I have tried it about 7 times. Each time it crashes no matter what I try (quitting, restarting, Shutting Computer down).
Process:
-I use the Elliptical Marquee Tool to make a selection
-I press "Q" (Toggle Edit Selection Edit As Layer)
-I select the Move Tool
-I attempt to use the Move Tool to move the selection, but as soon as I click the selection Affinity Photo immediately closes.
-Upon reopening Affinity Photo, it give a notice that Affinity Photo Unexpectedly Quit
*Please note that when using the Move Tool it will allow me to move the edges of the selection by moving the four points, but as soon as you click on the entire selection to move it at once, it will crash.*
System/Program Versions:
Affinity Photo Version 1.8.3
Mac OS Catalina Version 10.15.4