avoid trapping in case if the image is NIL

git-svn-id: https://svn.inf.ethz.ch/svn/lecturers/a2/trunk@7132 8c9fc860-2736-0410-a75d-ab315db34111

source/WMWindowManager.Mod

@@ -511,7 +511,7 @@ TYPE
 
 		PROCEDURE SetCanvasGenerator*(canvasGen:Graphics.CanvasGenerator);
 		BEGIN{EXCLUSIVE}
-			SELF.canvasGen:=canvasGen; canvas:=canvasGen(img); 
+			SELF.canvasGen:=canvasGen; IF img # NIL THEN canvas:=canvasGen(img); END;
 			IF manager # NIL THEN manager.AddVisibleDirty(SELF, bounds) END
 		END SetCanvasGenerator;
 		
@@ -603,8 +603,8 @@ TYPE
 		PROCEDURE SetCanvasGenerator*(canvasGen:Graphics.CanvasGenerator);
 		BEGIN
 			SELF.canvasGen:=canvasGen; 
-			visibleCanvas:=canvasGen(img);
-			canvas:=canvasGen(backImg); 
+			IF img # NIL THEN visibleCanvas:=canvasGen(img); END;
+			IF backImg # NIL THEN canvas:=canvasGen(backImg); END;
 			IF manager # NIL THEN manager.AddVisibleDirty(SELF, bounds) END
 		END SetCanvasGenerator;