Hi Stefano,
welcome to the ROOT forum!
Please check out this post about posting code on the forum – and feel free to edit your message so it’s better readable 
There should be one of the “candidate constructor not viable” error lines that mentions the constructor you want (it’s missing from your copy-paste): it’s the constructor with two arguments. The error message should then tell you why the compiler can’t use that constructor in that place.
Meanwhile let me see if I can reproduce on my machine with the files you shared… (EDIT: note that you can upload .C and .h files to the forum directly with the upload button in the edit bar)