fix(loader): fix npe in FileLine

hugegraph-loader/src/main/java/org/apache/hugegraph/loader/reader/file/FileLineFetcher.java

@@ -168,7 +168,14 @@ public Line fetch() throws IOException {
             if (this.needSkipLine(rawLine) || this.checkMatchHeader(rawLine)) {
                 continue;
             }
-            return this.parser.parse(this.source().header(), rawLine);
+            String[] header = this.source().header();
+            if (header == null) {
+                throw new LoadException("Header is null when parsing line at offset %s, " +
+                                        "this indicates a concurrency issue or initialization failure",
+                                        this.offset());
+            }
+
+            return this.parser.parse(header, rawLine);
         }
     }
 
@@ -230,9 +237,15 @@ private boolean checkMatchHeader(String line) {
             return false;
         }
 
-        assert this.source().header() != null;
+        String[] header = this.source().header();
+        if (header == null) {
+            LOG.warn("Header is null when checking match for line at offset {}, " +
+                     "this should not happen in normal cases", this.offset());
+            return false;
+        }
+
         String[] columns = this.parser.split(line);
-        return Arrays.equals(this.source().header(), columns);
+        return Arrays.equals(header, columns);
     }
 
     private static BufferedReader createBufferedReader(InputStream stream,

hugegraph-loader/src/main/java/org/apache/hugegraph/loader/reader/file/FileReader.java

@@ -104,6 +104,11 @@ public List<InputReader> split() {
 
         this.fetcher = this.createLineFetcher();
         this.fetcher.readHeaderIfNeeded(readableList);
+        if (this.source().format().needHeader() && this.source().header() == null) {
+            throw new InitException("Failed to initialize header for file source '%s'. " +
+                                    "Header is required but was not read successfully.",
+                                    this.source);
+        }
 
         this.readables = readableList.iterator();
         List<InputReader> readers = new ArrayList<>();

hugegraph-loader/src/test/java/org/apache/hugegraph/loader/test/functional/FileLoadTest.java

@@ -1197,9 +1197,7 @@ public void testMultiFilesHaveHeader() {
                 "-s", configPath("multi_files_have_header/schema.groovy"),
                 "-g", GRAPH,
                 "-h", SERVER,
-                "--test-mode", "true",
-                // FIXME: Set parser-threads to 1 because values > 1 currently trigger a NullPointerException (NPE).
-                "--parser-threads", "1"
+                "--test-mode", "true"
         };
         loadWithAuth(args);
 
@@ -1632,9 +1630,7 @@ public void testFilterPathBySuffix() {
                 "-s", configPath("filter_path_by_suffix/schema.groovy"),
                 "-g", GRAPH,
                 "-h", SERVER,
-                "--test-mode", "true",
-                // FIXME: Set parser-threads to 1 because values > 1 currently trigger a NullPointerException (NPE).
-                "--parser-threads", "1"
+                "--test-mode", "true"
         };
         loadWithAuth(args);