don't start with nullptr

GUI/Model/Device/InstrumentsSet.cpp

@@ -66,9 +66,8 @@ void InstrumentsSet::readFrom(QXmlStreamReader* r)
         // instrument
         if (tag == Tag::Instrument) {
             PolyItem<InstrumentsCatalog> p;
-            p.setCertainItem(nullptr);
+            p.readFrom(r);
             m_instruments.emplace_polyitem_back(std::move(p));
-            m_instruments.back().readFrom(r);
             XML::gotoEndElementOfTag(r, tag);
 
             // selected index