Yellow tanks give up if blocked

js/game.js

@@ -966,6 +966,11 @@ export class Level {
             return;
         let all_blocked = true;
         for (let [i, direction] of direction_preference.entries()) {
+            if (direction === null) {
+                // This actor is giving up!  Alas.
+                actor.decision = direction;
+                break;
+            }
             if (typeof direction === 'function') {
                 // Lazy direction calculation (used for walkers)
                 direction = direction();

js/tiletypes.js

@@ -1861,12 +1861,7 @@ const TILE_TYPES = {
         },
         movement_speed: 4,
         decide_movement(me, level) {
-            if (level.yellow_tank_decision) {
-                return [level.yellow_tank_decision];
-            }
-            else {
-                return null;
-            }
+            return [level.yellow_tank_decision, null];
         }
     },
     blob: {