avoid crash on show render objects if there are none

Renderer.hh

@@ -86,7 +86,7 @@ public:
 public slots:
   QString version() { return QString("1.0"); };
 
-  QString renderObjectsInfo(bool _outputShaderInfo) { return dumpCurrentRenderObjectsToString(&sortedObjects_[0],_outputShaderInfo); };
+  QString renderObjectsInfo(bool _outputShaderInfo) { return sortedObjects_.empty() ? "" : dumpCurrentRenderObjectsToString(&sortedObjects_[0],_outputShaderInfo); };
 
 
 private slots: