Now if there is a wrong XPath expression, the mapper shown a worning message dialog. 
It seems not enough because of it can be not obvious for users anyway. 
It is necessary to show such wrong XPath expressions someway in the mapper. 

For example the wrong XPathes can be analyzed and corresponding "wrong" nodes can be added to source or target tree.
Such nodes can be shown in red color to highlight its difference from ordinary schema nodes. Unfortunately, it's not so
simple solution. 

Another suggestion is to have a special tree node (for example in target tree) with name "Wrong XPath expressions". To
such node a string literal vertex is linked. This literal contains text of the wrong expression.

Other ideas are welcomed.