With commit 70f3384 interface of TBuffer::WriteStdString / TBuffer::ReadStdString was changed. Before std::string* was used as argument, now std::string&.
But this does not work in the case of zero-pointer.
Means when following field just zero:
ROOT just crashes when try to write such object into the file.
Also following statement in TBufferFile::WriteStdString wrong:
if (s==0) return;