UnboundLocalError: local variable 'ppg' referenced before assignment

**Summary[问题简述（一句话）]**
上传自己的wav文件报UnboundLocalError: local variable 'ppg' referenced before assignment

**Env & To Reproduce[复现与环境]**
描述你用的环境、代码版本、模型
Windows 10 ,python3.8 按照readme.txt安装成功 ，执行python demo_toolbox.py -vc -d "E:\3DData\sounddata\"
**Screenshots[截图（如有）]**
出错信息：
Traceback (most recent call last):
File "E:\1JSONPro\MBIRD\control\toolbox\__init__.py", line 141, in
func = lambda: self.convert() or self.vocode()
File "E:\1JSONPro\MBIRD\control\toolbox\__init__.py", line 378, in convert
min_len = min(ppg.shape[1], len(lf0_uv))
UnboundLocalError: local variable 'ppg' referenced before assignment
Traceback (most recent call last):
File "E:\1JSONPro\MBIRD\control\toolbox\__init__.py", line 141, in
func = lambda: self.convert() or self.vocode()
File "E:\1JSONPro\MBIRD\control\toolbox\__init__.py", line 378, in convert
min_len = min(ppg.shape[1], len(lf0_uv))
UnboundLocalError: local variable 'ppg' referenced before assignment
Traceback (most recent call last):
File "E:\1JSONPro\MBIRD\control\toolbox\__init__.py", line 141, in
func = lambda: self.convert() or self.vocode()
File "E:\1JSONPro\MBIRD\control\toolbox\__init__.py", line 378, in convert
min_len = min(ppg.shape[1], len(lf0_uv))
UnboundLocalError: local variable 'ppg' referenced before assignment

Reproduce the failure with `python demo_toolbox.py -vc -d "E:\3DData\sounddata\"` on the reported Windows/Python environment. Read `control/toolbox/__init__.py` around the `convert` method and line 378, then trace the paths that lead to `ppg.shape[1]`; done means the uploaded WAV no longer raises `UnboundLocalError` during conversion.

Written by the indexing model from the issue text.