fix: Use TKey::ReadObject<T> instead of ReadObj

[ChristianTackeGSI]

TKey::ReadObj returns an owning pointer. Using dynamic_cast and ignoring the "not castable" case can leak memory.

Also one std::unique_ptr

See: https://root-forum.cern.ch/t/possible-leak-with-dynamic-cast-and-tkey-readobj/56799 See: https://github.com/root-project/root/pull/13931

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Checklist:

• [X] Followed the Contributing Guidelines