Error in implicit conversion. Cannot convert null object

when i use write cell activity directly, it post an error:Error in implicit conversion. Cannot convert null object.
but if i use the write cell activity with Excel Application Scope, it is OK.
So what cause this error?
error

The write cell with the excel application scope is a bit more stable. Try the other write cell using a workbook path with English only filename as the filename might have confused the write cell coding