Updates to fix multiplayer bug found in tutorial. (#23)

These changes match those found in the Docs side with PR clockworklabs/spacetime-docs#169

Author: rekhoff

demo/Blackholio/server-csharp/Lib.cs

@@ -350,7 +350,13 @@ public static void MoveAllPlayers(ReducerContext ctx, MoveAllPlayersTimer timer)
 		//Handle player input
 		foreach (var circle in ctx.Db.circle.Iter())
 		{
-			var circle_entity = ctx.Db.entity.entity_id.Find(circle.entity_id) ?? throw new Exception("Circle has no entity");
+			var check_entity = ctx.Db.entity.entity_id.Find(circle.entity_id);
+			if (check_entity == null)
+			{
+				// This can happen if the circle has been eaten by another circle.
+				continue;
+			}
+			var circle_entity = check_entity.Value;
 			var circle_radius = MassToRadius(circle_entity.mass);
 			var direction = circle_directions[circle.entity_id];
 			var new_pos = circle_entity.position + direction * MassToMaxMoveSpeed(circle_entity.mass);

demo/Blackholio/server-rust/src/lib.rs

@@ -406,7 +406,12 @@ pub fn move_all_players(ctx: &ReducerContext, _timer: MoveAllPlayersTimer) -> Re
 
     // Handle player input
     for circle in ctx.db.circle().iter() {
-        let mut circle_entity = ctx.db.entity().entity_id().find(&circle.entity_id).unwrap();
+        let circle_entity = ctx.db.entity().entity_id().find(&circle.entity_id);
+        if !circle_entity.is_some() {
+            // This can happen if a circle is eaten by another circle
+            continue;
+        }
+        let mut circle_entity = circle_entity.unwrap();
         let circle_radius = mass_to_radius(circle_entity.mass);
         let direction = *circle_directions.get(&circle.entity_id).unwrap();
         let new_pos =