Fix warning if building without 3d

(AKA hurry up Fedora 27, I want Qt 5.9 already)

Author: nyalldawson

src/app/qgisapp.cpp

@@ -10031,6 +10031,7 @@ Qgs3DMapCanvasDockWidget *QgisApp::createNew3DMapCanvasDock( const QString &name
   return map3DWidget;
 #else
   Q_UNUSED( name );
+  return nullptr;
 #endif
 }