[RF] Correctly clone TTree in RooTreeDataStore::loadValues

Follows up on #6452.

Author: guitargeek

roofit/roofitcore/src/RooTreeDataStore.cxx

@@ -366,11 +366,8 @@ void RooTreeDataStore::createTree(RooStringView name, RooStringView title)
 void RooTreeDataStore::loadValues(const TTree *t, const RooFormulaVar* select, const char* /*rangeName*/, Int_t /*nStart*/, Int_t /*nStop*/)
 {
   // Make our local copy of the tree, so we can safely loop through it.
-  // We need a custom deleter, because if we don't deregister the Tree from the directory
-  // of the original, it tears it down at destruction time!
-  auto deleter = [](TTree* tree){tree->SetDirectory(nullptr); delete tree;};
-  std::unique_ptr<TTree, decltype(deleter)> tClone(static_cast<TTree*>(t->Clone()), deleter);
-  tClone->SetDirectory(t->GetDirectory());
+  std::unique_ptr<TTree> tClone{static_cast<TTree*>(const_cast<TTree*>(t)->CloneTree())};
+  tClone->SetDirectory(nullptr);
 
   // Clone list of variables
   RooArgSet sourceArgSet;