core: type check before adding langs data

packages/hydrooj/package.json

@@ -1,6 +1,6 @@
 {
     "name": "hydrooj",
-    "version": "3.3.17",
+    "version": "3.3.18",
     "bin": "bin/hydrooj.js",
     "main": "src/loader",
     "module": "src/loader",

packages/hydrooj/src/handler/problem.ts

@@ -306,9 +306,9 @@ export class ProblemDetailHandler extends ProblemHandler {
             if (!ddoc || !pdoc) throw new ProblemNotFoundError(this.pdoc.reference.domainId, this.pdoc.reference.pid);
             this.pdoc.config = pdoc.config;
         }
-        if (ddoc.langs) {
-            (this.pdoc.config as ProblemConfig).langs = intersection(
-                (this.pdoc.config as ProblemConfig).langs || ddoc.langs.split(','),
+        if (ddoc.langs && typeof this.pdoc.config !== 'string') {
+            this.pdoc.config.langs = intersection(
+                this.pdoc.config.langs || ddoc.langs.split(','),
                 ddoc.langs.split(','),
             );
         }