Obviously, cancelling this event would prevent other mods in-screen texts, if they planned to add those in such way.
On the Battlegear topic, you should have asked directly to the mod topic, you just are lucky I came in here.
Anyway, this crash shouldn't happen if you are listening to RenderGameOverlayEvent.Pre and using a newer Battlegear version.
Also, ElementType is an enum. You can do
if(event.type == RenderGameOverlayEvent.ElementType.TEXT)
That would work the same.