Disallow null pointers to diffractometer instruments
All low-level data-readers assume a valid (raw) pointer to an
instance of diffractometer instrument (see eg., core/raw/IDataReader
or core/loader/HDF5MetaDataReader
). Therefore, this logical
requirement must be imposed at a high level to prevent undefined
behaviour.
This validity condition is currently verified in the constructor of
core/experiment/DataHandler
and an exception is thrown in case of an
invalid instrument.
The exception is caught in the GUI session, gui/models/Session::loadData
.
Resolves issue 224