Re #6198. Update InstrumentView usage of mapping method.

I restored the documented exception throwing behaviour after checking that
every place that calls the method catches the exception. Not sure when/why
that behaviour was changed, but this avoids the risk of accessing an
invalid iterator.

Code/Mantid/MantidPlot/src/Mantid/InstrumentWidget/InstrumentActor.cpp

@@ -147,7 +147,6 @@ m_sampleActor(NULL)
 InstrumentActor::~InstrumentActor()
 {
   saveSettings();
-  delete m_detid2index_map;
 }
 
 /** Used to set visibility of an actor corresponding to a particular component
@@ -314,7 +313,13 @@ IDetector_const_sptr InstrumentActor::getDetector(size_t i) const
  */
 size_t InstrumentActor::getWorkspaceIndex(Mantid::detid_t id) const
 {
-    return (*m_detid2index_map)[id];
+  auto mapEntry = m_detid2index_map.find(id);
+  if ( mapEntry == m_detid2index_map.end() )
+  {
+    throw Kernel::Exception::NotFoundError("Detector ID not in workspace",id);
+  }
+
+  return mapEntry->second;
 }
 
 /**

Code/Mantid/MantidPlot/src/Mantid/InstrumentWidget/InstrumentActor.h

@@ -185,8 +185,8 @@ class InstrumentActor: public QObject, public GLActor
   /// Flag to show the guide and other components. Loaded and saved in settings.
   bool m_showGuides;
 
-  /// Pointer to the workspace's detector ID to workspace index map
-  Mantid::detid2index_map *m_detid2index_map;
+  /// The workspace's detector ID to workspace index map
+  Mantid::detid2index_map m_detid2index_map;
 
   /// All det ids in the instrument in order of pickIDs, populated by Obj..Actor constructors
   mutable std::vector<Mantid::detid_t> m_detIDs;