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Merge PDF - Object reference not set to an instance of an object


I am trying to figure out why my desktop flow is unable to merge a list of PDFs in a folder. It seems to only happen with the files the flow creates. I tried to make a standalone flow that just combined PDFs to test it out. It failed the first time, then immediately worked the second run. 

Trying a simple Retry with any amount of waiting will continue to fail in the flow.

Microsoft.Flow.RPA.Desktop.Modules.SDK.ActionException: Failed to merge PDF files:
*Insert 346 PDF Paths*
---> System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.Flow.RPA.Desktop.Modules.PDF.Actions.PDFium.PDFium.FPDF_SaveAsCopy(IntPtr doc, FPDF_FILEWRITE writer, Int32 flags)
at Microsoft.Flow.RPA.Desktop.Modules.PDF.Actions.MergePDFFilesAction.Execute(ActionContext context)
--- End of inner exception stack trace ---
at Microsoft.Flow.RPA.Desktop.Modules.PDF.Actions.MergePDFFilesAction.Execute(ActionContext context)
at Microsoft.Flow.RPA.Desktop.Robin.Engine.Execution.ActionRunner.Run(IActionStatement statement, Dictionary`2 inputArguments, Dictionary`2 outputArguments)




The basic function of this is to clean 1 large document and batch processed with Irfranview. Then to recombine the split files to be worked manually. 


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