diff --git a/src/error_gui.cpp b/src/error_gui.cpp
--- a/src/error_gui.cpp
+++ b/src/error_gui.cpp
@@ -235,7 +235,7 @@ public:
 		int scr_bot = GetMainViewBottom() - 20;
 
 		Point pt = RemapCoords(this->position.x, this->position.y, GetSlopePixelZOutsideMap(this->position.x, this->position.y));
-		const Viewport *vp = FindWindowById(WC_MAIN_WINDOW, 0)->viewport;
+		const Viewport *vp = GetMainWindow()->viewport;
 		if (this->face == INVALID_COMPANY) {
 			/* move x pos to opposite corner */
 			pt.x = UnScaleByZoom(pt.x - vp->virtual_left, vp->zoom) + vp->left;