A bug when llm_description got None response from LLM in PptxConverter

[wangxb12]

Got below error when converting a .pptx file.

Error when converting the document: File conversion failed after 1 attempts:

• PptxConverter threw TypeError with message: sequence item 0: expected str instance, NoneType found

After investigation the llm_description got None response from LLM in line 120 of _pptx_converter.py and it will cause this issue in line 139.