Addendum to matroid tree-width

Hliněný and Whittle have shown that the traditional tree-width notion of a graph can be defined without an explicit reference to vertices, and that it can be naturally extended to all matroids. Unfortunately their original paper [P. Hliněný, G. Whittle, Matroid tree-width, European J. Combin. 27 (2006) 1117–1128], as pointed out by Isolde Adler in 2007, contained some incorrect arguments. It is the purpose of this addendum to correct the affected proofs. (All the theorems and results of the original paper remain valid.)