Fix the NPE bug from breaking an opened shulker

src/main/java/us/mcdevs/spigot/nestedshulkers/NestedShulkers.kt

@@ -99,9 +99,14 @@ class NestedShulkers : JavaPlugin(), Listener {
         }
         Bukkit.getScheduler().runTask(this, Runnable {
             if (temp.viewers.isEmpty()) {
-                shulker.storageContents = temp.storageContents
-                shulkers.remove((shulker.holder!! as ShulkerBox).location)
-                closeShulker(shulker)
+                //Catch a weird NPE from breaking the block
+                shulker.let {
+                    it.storageContents = temp.storageContents
+                    (it.holder as? ShulkerBox)?.location?.apply {
+                        shulkers.remove(this)
+                        closeShulker(it)
+                    }
+                }
             }
         })
     }
@@ -109,8 +114,7 @@ class NestedShulkers : JavaPlugin(), Listener {
     @EventHandler
     private fun onShulkerBreak(event: BlockBreakEvent) {
         val shulker = shulkers[event.block.location] ?: return
-        shulker.viewers.forEach { it.closeInventory() }
-        closeShulker(shulker)
+        ArrayList(shulker.viewers).forEach { it.closeInventory() }
     }
 
     /**