ProjectExplorer: Nodes: Do not makeEmpty() as part of buildTree()

Do not call makeEmpty() as part of buildTree(). That makes it
impossible to combine buildTree with manual tree setup.

Change-Id: If0a0d9432fe39870917a6ba31594e8dcd6d31868
Reviewed-by: hjk <hjk@qt.io>

src/plugins/pythoneditor/pythoneditorplugin.cpp

@@ -618,6 +618,7 @@ void PythonProject::refresh()
         const QString displayName = baseDir.relativeFilePath(f);
         return new PythonFileNode(FileName::fromString(f), displayName);
     });
+    rootProjectNode()->makeEmpty();
     rootProjectNode()->buildTree(fileNodes);
 
     emit parsingFinished();