Visual Mode: fixed a crash when toggling visual vertex slope picking and the...

Visual Mode: fixed a crash when toggling visual vertex slope picking and the map contains double-sided linedefs without a front sidedef. Fixes #928

Source/Plugins/BuilderModes/VisualModes/VisualVertexSlope.cs

@@ -37,7 +37,7 @@ namespace CodeImp.DoomBuilder.VisualModes
 
 			angle = line.GetAngle();
 
-			if (ld.Front.Sector != s)
+			if (ld.Front?.Sector != s)
 				clockwise = !clockwise;
 		}
 	}