Re #5731. Made the test on instrument emptiness a little smarter.

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

@@ -100,16 +100,22 @@ m_sampleActor(NULL)
   /// Cache a map (actually a vector) to workspace indexes.
   shared_workspace->getDetectorIDToWorkspaceIndexVector(m_id2wi_vector, m_id2wi_offset, false);
 
+  auto instrument = getInstrument();
+
   // If the instrument is empty, maybe only having the sample and source
-  if (getInstrument()->nelements() < 3)
+  int nelements = instrument->nelements();
+  if (nelements == 0 ||
+      nelements == 1 && (instrument->getSource() || instrument->getSample()) ||
+      nelements == 2 && instrument->getSource() && instrument->getSample()
+    )
   {
     QMessageBox::warning(NULL,"MantidPlot - Warning","The instrument is probably empty","OK");
   }
 
   // this adds actors for all instrument components to the scene and fills in m_detIDs
-  m_scene.addActor(new CompAssemblyActor(*this,getInstrument()->getComponentID()));
+  m_scene.addActor(new CompAssemblyActor(*this,instrument->getComponentID()));
 
-  FindComponentVisitor findVisitor(getInstrument()->getSample()->getComponentID());
+  FindComponentVisitor findVisitor(instrument->getSample()->getComponentID());
   accept(findVisitor);
   const ObjComponentActor* samplePosActor = dynamic_cast<const ObjComponentActor*>(findVisitor.getActor());