NPC Store = npcs.closest("Magic Store owner");
is causing it. initialize it when you need to in onLoop(). like this: 
NPC store;
public int onLoop() {
 if (needToInteractWithStore) {
  store = npc.closest("Name");
   if (store != null && store.interact("action")) {
     // conditional sleep till widget or interface is open/not null
  } 
 }
return 69;
}