hey Luke, good to hear from ya,
Are you using the latest build? (i addressed very similar errors over the past few months)
a missing RMP file shouldn't matter; iirc. It's highly unlikely to be related to the error ... MainView doesn't care about routes.
if the newest release (i changed the download to github itself, btw) is borking, will need a link to your mapfile etc ...
from the stacktrace i notice this is a pseudo-known .net issue ... MainViewOverlay.DrawTile() should be calling a function MainViewOverlay.DrawSprite() before the Graphics.DrawImage() call ...
while i've seen it before, no one seems to have a good explanation of it,
anyway, try the latest and/or perhaps get in touch by PM so we can poke this.