fix: bug

src/features/mark/scoring/MarkBoardTrack.vue

@@ -373,7 +373,7 @@ function cancelProblem() {
   if (!markStore.currentQuestion) return;
 
   markStore.currentQuestion.problem = false;
-  markStore.currentQuestion.problemType = "";
+  markStore.currentQuestion.problemType = null;
   markStore.currentQuestion.problemRemark = "";
 }