[BUG] Execute experiment throw null pointer exception

### 📋 CheckList

- [x] I have searched existing issues to avoid duplicates
- [x] I am using a recently maintained version of Coze Loop
- [x] I have provided all required information
- [x] I understand this is a bug report and not a feature request
- [x] I have submitted this report in English (otherwise it will not be processed)

### 🐛 Bug Description

Execute experiment throw null pointer exception


### 🔄 Steps to Reproduce

1.create Evaluation set
2.create evaluators
3.create experiments

### ✅ Expected Behavior

experiments runs successful

### ❌ Actual Behavior

experiments runs fails

### 🚨 Severity

None

### 🔧 Component

None

### 💻 Environment

_No response_

### 🔧 Go Environment

_No response_

### 📋 Logs

```shell

```

### 📝 Additional Context

_No response_