fix: guard null in GlobalVariableStore lock/unlock

$gc()/$g() JS APIs call lock() on keys never registered via putSync(), causing NPE; throw IllegalArgumentException with clear message instead

Signed-off-by: Nico Piel <nico.piel@hotmail.de>

Author: NicoPiel

server/src/com/mirth/connect/server/util/GlobalVariableStore.java

@@ -69,11 +69,19 @@ public String toString() {
     // ##### SYNC METHODS #####
 
     public void lock(String key) {
-        globalVariableSyncMap.get(key).getLock().lock();
+        SyncObject obj = globalVariableSyncMap.get(key);
+        if (obj == null) {
+            throw new IllegalArgumentException("Sync key not found: " + key);
+        }
+        obj.getLock().lock();
     }
 
     public void unlock(String key) {
-        globalVariableSyncMap.get(key).getLock().unlock();
+        SyncObject obj = globalVariableSyncMap.get(key);
+        if (obj == null) {
+            throw new IllegalArgumentException("Sync key not found: " + key);
+        }
+        obj.getLock().unlock();
     }
 
     public boolean containsKeySync(String key) {