It's a corner case but still should be fixed as users don't know if the tool is not working or if there is no data.

If I use the -d dump_blocks options, the browser opens a page with no plots at all.
If I use the -d dump_pages options, I get the following error:

Exception: Expected the line starts with 'Root:' but found ''

For both commands, it would be great to have a message saying there is no data to parse.