Excel files: object reference not set to an instance of an object

Hi,

 I'm working in Studio 19 with Excel files provided by the customer. The TM works fine but I get an error message when I try to generate the final target file and I don't know why.

I've been looking for answers on various forums and somebody suggested changing the order of the file type filters, but I don't know what to change.

Any suggestions gratefully welcomed!

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