Check if brewingStand == null

Fixes #1804

src/main/java/com/gmail/nossr50/runnables/skills/AlchemyBrewTask.java

@@ -57,7 +57,7 @@ public class AlchemyBrewTask extends BukkitRunnable {
 
     @Override
     public void run() {
-        if (player == null || !player.isValid() || brewingStand.getType() != Material.BREWING_STAND) {
+        if (player == null || !player.isValid() || brewingStand == null || brewingStand.getType() != Material.BREWING_STAND) {
             if (Alchemy.brewingStandMap.containsKey(brewingStand)) {
                 Alchemy.brewingStandMap.remove(brewingStand);
             }