I get this error with firebug
for a grid with subgrids :
oTidyBrowser is not defined
[Break on this error] if (r._childIndexes)
oTidyBrowser is not defined
Such name never used in code of grid, please check your custom code.
>>[Break on this error] if (r._childIndexes)
It hard to say, but most probably you have used incorrect ID or not existin Index in some command.
If problem still occurs for you - please provide any kind of sample where it can be reconstructed.