Did you click through quite quickly? While working on some PRs I noticed this issue when I clicked through rapidly. Going back to the start and clicking continue after a second or two allowed the window to render.
Might be your issue 
Windows 10 21h2
Mailspring 1.10.5
This bug still happens sometimes when I close Mailspring without closing an open mail. Iām able to close the white screen and open Mailspring afterwards though