Intentionally left blank.The document root element is not supported by the visual designer.

if ur xaml is not blank and has controls
close VS , open VS and build the whole solution.
the error wil vanish :)

"Invalid index. (Exception from HRESULT: 0x8002000B (DISP_E_BADINDEX))"

That is because the parameters (or parameters names) you set in your code are different from defined parameters in your report