Just to confirm, you are referring to map_gen\maps\diggy\config.lua:
enable_stress_grid = true? or enable_mask_debug = true? The stress grid works for me but enabling the mask debug crashes. It indicates arg surface is a null value line 445 in diggy_cave_collapse.lua. A value for surface should be set from RS.get_surface() on line 439. That function in redmew_surface.lua has a comment that it is not safe to call outside of events which may be relevant. But I am not smart enough to fix it. Maybe someone else can. If the reason you are trying to turn on debugging for cave collapses in the first place is due to diggy cave collapses not working right in 0.18, Make sure you have the latest version as there was a diggy 0.18 patch merged just this morning. Even though the updates were just for the particle system, we were seeing some weird side effects in unexpected collapses, biters not spawning or bot mining issues before the patch. I suspect the display of the debug mask may have been broken for a while as these files weren't touched during the update of diggy to 0.18.

I do have an incorrect behaviour of the collapse. Robots not been able to mine rocks .

Yes this was an issue when running diggy code prior to today (the RedMew S6 server was testing the 0.18 update for about a week) but the actual update ( PR #1032 ) was not merged until about 7 hours ago. If you re-download Refactorio/Redmew now it should work now.

Back to report !

The robots can correctly mine rocks now.
The stress grid is displayed correctly with the correct values.
The mask debug still gives a fatal error.

Thank you, have a nice day.

It may be some time before there is an approved fix for the diggy mask debug display issue. If you want to view the mask debug information before then I found an temporary fix in diggy_cave_collapse.lua that worked for me. I moved the code block (lines 438-448) and inserted it between lines 428 and 429. This makes the mask info display at the origin right after you destroy your first rock. I am pretty new here, and it isn't a very elegant solution but it works.

The error happens because the surface doesn't exist yet when the function is called the first time. I've done some testing too, but it doesn't seem to be updating. To be honest I'm not really sure of what the mask debugging option does.

His quick fix stops it from crashing.

Sounds good, I might push a temporary fix for the crashing, although I'm not sure the debugging option works at all.

The displayed grid is correct and accurate.

The displayed grid is correct and accurate.

Could you provide an image for reference? Thanks :D Got one already