Since the LocalFunction stores a pointer to the
DiscreteGlobalBasisFunction, calling localFunction(...)
with a temporary leads to a dangling reference.

This happens, e.g., when passing a temporary
DiscreteGlobalBasisFunction to VTKWriter::addVertexData(...)
because the latter does not store its argument, but
only the obtained LocalFunction.

Thanks to @max.kahnt for pointing out the issue.