if there is not safe path the application will now show a message instead of crashing

Buhnenrennen/GraphicGenerator.cs

@@ -121,12 +121,15 @@ namespace Buhnenrennen
                 }
 
                 TimedGroyne safePathSub = safePath;
-
-                do
+                if ( safePath != null )
                 {
-                    graphics.ConnectRectangles( new Pen( safePathBrush, borderWidth ), groynePositions[ safePathSub.Groyne ], groynePositions[ safePathSub.ParentGroyne.Groyne ], borderWidth );
-                    safePathSub = safePathSub.ParentGroyne;
-                } while ( safePathSub.ParentGroyne != null );
+                    do
+                    {
+                        graphics.ConnectRectangles( new Pen( safePathBrush, borderWidth ), groynePositions[ safePathSub.Groyne ], groynePositions[ safePathSub.ParentGroyne.Groyne ], borderWidth );
+                        safePathSub = safePathSub.ParentGroyne;
+                    } while ( safePathSub.ParentGroyne != null );
+                }
+                else ColorMessageBox.Show( "Das gegebene Scenario enthält keine sichere Route für Minnie" );
 
                 foreach( var groyne in groyneBorderBrush.Keys )
                 {