Hi Fabiano,
you already given the answer to your question by yourself:
"The only step I didn't finished was to create a custom handler class inherinting from CL_MDG_BS_ECC_HANDLER, which executes the mapping."
Without the mapping you call check w/o data. This usually leads to error messages as shown in your screenshot.
If you don't follow the guide completely you can expect a working extension.
Best regards
Michael