Hi! I had the same problem, I created an interface on C++ and added in the editor to a Widget Blueprint, but the cast return a null.
IResultsInterface* sr = Cast<IResultsInterface>(object);
if (sr) sr->SetSumaryData(data); //fail
The solution was to use the static version of the function:
if (object->GetClass()->ImplementsInterface(IResultsInterface::StaticClass()))
{
IResultsInterface::Execute_SetSumaryData(b, data);
}
Engine was 5.2.