[fixed] delete material

[micheus]

Thanks for report it.
It looks like the Outliner takes a little longer to be updated and you end up by trying to delete a second time. And if I remember your screenshot, that project has a lot of material/objects.

I delete one by one materials assigned to my object and for some step I get: ...

just curious...
Did you delete all (or most) of them? Because it seems to be faster you just select the object and set its material to Default; Save your project and Reload it.