When a table has a __metatable key which returns something that is not a table (protected metatable usually return a string here), you have this error.

you can fix very quickly changing line 414 (function ViragDevTool:NewMetatableNode) from
if mt then
to
if type(mt)=="table"

A nicer way would be displaying the value (but this require digging in your code more than I had time to).

Btw, this tool is priceless, many thanks for developing it

Just added a pull request. #2