solve concurrency issue in WriteDocsToDatabase

writeQueue.poll gives null when empty (which is possible without snychronizing the method call). This leads to a crash in writeData

deidentifier-pipeline/src/main/java/org/ratschlab/deidentifier/workflows/WriteDocsToDatabase.java

@@ -51,7 +51,7 @@ public void doneHook() {
         }
     }
 
-    private void flushQueue() throws SQLException {
+    private synchronized void flushQueue() throws SQLException {
         List<Pair<String, Map<Object, Object>>> lst = new ArrayList<>();
 
         while (!writeQueue.isEmpty()) {