Instrument View: Selecting a detector that isn't in the workspace masks the first spectrum

[mantid-builder]

Original Reporter: Russell Taylor

i.e. workspace index 0 is masked if a detector selected does not have

This is because the map InstrumentActor::m_id2wi_map is in several places being accessed via operator[] in what should just be a read, so it's adding entries with 0 as the second member of the pair when called with detectors that aren't in it.

[mantid-builder]

This issue was originally trac ticket 3698