I think that fixes the bug. Only SetUnit if mouseover exists.

diff --git a/StarTip.lua b/StarTip.lua
index 2f61b58..3af02f8 100644
--- a/StarTip.lua
+++ b/StarTip.lua
@@ -626,7 +626,9 @@ end
 local throttleTimer
 local lastTime = GetTime()
 local function endThrottle()
-	StarTip.OnTooltipSetUnit()
+	if UnitExists(StarTip.unit or "mouseover") then
+		StarTip.OnTooltipSetUnit()
+	end
 end

 function StarTip.OnTooltipSetUnit(...)