[tree] Check that chain element is a TTree.
[tree] Check that chain element is a TTree.

root-mirror/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,karies/root,karies/root,root-mirror/root,karies/root