The error that you are seeing is caused by the inputobject being empty in the java action mapping.
Below is an explanation of how to work with the excel exporter with a simple example.
The input object is an entity that you create and associate the data you want to export to. Let's say you have an entity Person containing persons with a couple of attributes describing the person. The second entity you have is an Organisation. Every person record has an association to 1 organisation. If you want to export all the persons for a specific organisation your template input object is the organisation entity. In the next steps (the sheet) you define the row object as the Person entity, with the reference to the input object the association between person and organisation. Then define the column data, the attributes of the Person entity. Now your template is ready.
The from the useme folder in the modeler copy the GenerateReport microflow and make sure that:
This is just an example, but you can of course create the input object in a microflow, retrieve the export values you need and associate them to the new input object (matching what you setup in the template), retrieve the template, create the filedocument and call the java action and download to create any kind of xls/csv export.
Hope this helps in understanding how to work with the excel exporter.