Hello !
I've noticed there is a kind of bug when i work with mask ans quick mask, here is what i did.
Loaded an image
Made a selection of the subject (worked)
Press Q to display quick mask (worked)
Click the mask button close to the adjustment in order to mask unnecessary area ...
... Affinity Photo crashed !
Retry the same with rasterized image but the same result.
Now when i apply mask with no quick mask displayed it works.
Question: it should be okay to apply mask with quick mask displayed, ain't it ?
Hope this will be corrected in coming Beta version.
Blessings