Enabling export in the sample HierarchicalGrid.aspx, provided WebGrid sample,  and using the latest release build for WebGrid 6 and WebUIFramework 3, build 218 and build 705 respectively, I could not replicate the issue you are reporting.

I have tried the export to numerous document type but still could not produce the error message.

I was invoking export client-side from a button outside the grid. After adding the name of the table the export works.

// Staffan

Export client side event will require at least 3 parameter:

• tblName ( string ) - The name of the table to be exported.

• exportType ( string ) - The type of the exported document, e.g: "HTML", "PDF".

• pageOrientation ( string ) - The page orientation of the exported document, e.g: "PORTRAIT", "LANDSCAPE".
  

var grid = ISGetObject('WebGrid1');
grid.ExportGrid("Customers", "PDF", "PORTRAIT");