Allow custom unusable-space blocker item (#4038)

* Allow the "unusable-space-block" to be a custom item (specified by bedrock identifier)

* add space for log error

* don't use ugly atomic reference

core/src/main/java/org/geysermc/geyser/util/InventoryUtils.java

@@ -208,12 +208,21 @@ public class InventoryUtils {
 
     private static ItemDefinition getUnusableSpaceBlockDefinition(int protocolVersion) {
         String unusableSpaceBlock = GeyserImpl.getInstance().getConfig().getUnusableSpaceBlock();
-        ItemMapping unusableSpaceBlockID = Registries.ITEMS.forVersion(protocolVersion).getMapping(unusableSpaceBlock);
-        if (unusableSpaceBlockID != null) {
-            return unusableSpaceBlockID.getBedrockDefinition();
-        } else {
+        ItemDefinition itemDefinition = null;
+
+        // looping through all the items to find the one with the specified Bedrock identifier
+        for (ItemDefinition definition : Registries.ITEMS.forVersion(protocolVersion).getItemDefinitions().values()) {
+            if (definition.getIdentifier().equals(unusableSpaceBlock)) {
+                itemDefinition = definition;
+                break;
+            }
+        }
+
+        if (itemDefinition == null) {
             GeyserImpl.getInstance().getLogger().error("Invalid value " + unusableSpaceBlock + ". Resorting to barrier block.");
             return Registries.ITEMS.forVersion(protocolVersion).getStoredItems().barrier().getBedrockDefinition();
+        } else {
+            return itemDefinition;
         }
     }