Fixed concurrent config modification issues

When multiple threads attempt to add nodes and reorder a config section
concurrency could cause a NPE in ConfigWriter

- Prevented the NPE
- Prevented concurrent modification between multiple node modification methods with a semaphore

Plan/common/src/main/java/com/djrapitops/plan/settings/config/ConfigNode.java

@@ -23,6 +23,7 @@
  */
 package com.djrapitops.plan.settings.config;
 
+import com.djrapitops.plan.utilities.UnitSemaphoreAccessLock;
 import org.apache.commons.lang3.StringUtils;
 
 import java.io.IOException;
@@ -40,6 +41,8 @@ import java.util.stream.Collectors;
  */
 public class ConfigNode {
 
+    protected final UnitSemaphoreAccessLock nodeModificationLock = new UnitSemaphoreAccessLock();
+
     protected final String key;
     protected ConfigNode parent;
 
@@ -128,8 +131,11 @@ public class ConfigNode {
         if (parent == null) {
             throw new IllegalStateException("Can not remove root node from a tree.");
         }
-        parent.childNodes.remove(key);
+        nodeModificationLock.enter();
         parent.nodeOrder.remove(key);
+        parent.childNodes.remove(key);
+        nodeModificationLock.exit();
+
         updateParent(null);
 
         // Remove children recursively to avoid memory leaks
@@ -149,8 +155,12 @@ public class ConfigNode {
      */
     protected ConfigNode addChild(ConfigNode child) {
         getNode(child.key).ifPresent(ConfigNode::remove);
+
+        nodeModificationLock.enter();
         childNodes.put(child.key, child);
         nodeOrder.add(child.key);
+        nodeModificationLock.exit();
+
         child.updateParent(this);
         return child;
     }
@@ -197,6 +207,7 @@ public class ConfigNode {
     }
 
     public void reorder(List<String> newOrder) {
+        nodeModificationLock.enter();
         List<String> oldOrder = nodeOrder;
         nodeOrder = new ArrayList<>();
         for (String childKey : newOrder) {
@@ -207,6 +218,7 @@ public class ConfigNode {
         // Add those that were not in the new order, but are in the old order.
         oldOrder.removeAll(nodeOrder);
         nodeOrder.addAll(oldOrder);
+        nodeModificationLock.exit();
     }
 
     /**

Plan/common/src/main/java/com/djrapitops/plan/settings/config/ConfigWriter.java

@@ -95,7 +95,10 @@ public class ConfigWriter {
         Map<String, ConfigNode> children = writing.childNodes;
         for (String key : writing.getNodeOrder()) {
             ConfigNode node = children.get(key);
-            if (node.value == null && node.nodeOrder.isEmpty()) {
+            // node is null:       Inconsistent config node state
+            // value is null:      Has no value (empty)
+            // nodeOrder is empty: Has no children
+            if (node == null || node.value == null && node.nodeOrder.isEmpty()) {
                 continue;
             }