Accommodate fields being possibly null

Author: pfroud

src/entityViewer.ts

@@ -96,7 +96,7 @@ namespace EntityViewer {
 
             let data: ListViewItem[] = [
                 sep('Entity'),
-                ["Id:", entity.id.toString()],
+                ["Id:", entity.id?.toString() ?? "null"],
                 ["Type:", entity.type.toString()],
                 ["Position:", `${entity.x}, ${entity.y}, ${entity.z}`]
             ];
@@ -113,8 +113,8 @@ namespace EntityViewer {
                         ["Sprite type", car.spriteType.toString()],
                         ["Num. of seats", car.numSeats.toString()],
                         ["Next car on train", car.nextCarOnTrain?.toString() ?? "null"],
-                        ["Next car on ride", car.nextCarOnRide.toString()],
-                        ["Previous car on ride", car.previousCarOnRide.toString()],
+                        ["Next car on ride", car.nextCarOnRide?.toString() ?? "null"],
+                        ["Previous car on ride", car.previousCarOnRide?.toString() ?? "null"],
                         ["Current station", car.currentStation.toString()],
                         ["", ""],
                         ["Mass:", car.mass.toString()],