No need to call expandAll from outside the model.

src/calamares/CalamaresApplication.cpp

@@ -138,7 +138,6 @@ CalamaresApplication::onPluginsReady()
         //TODO: move somewhere
         ProgressTreeModel* m = new ProgressTreeModel( this );
         ProgressTreeView::instance()->setModel( m );
-        ProgressTreeView::instance()->expandAll();
     });
 }
 

src/calamares/progresstree/ProgressTreeView.cpp

@@ -51,3 +51,11 @@ ProgressTreeView::~ProgressTreeView()
 {
 
 }
+
+
+void
+ProgressTreeView::setModel( QAbstractItemModel* model )
+{
+    QTreeView::setModel( model );
+    expandAll();
+}

src/calamares/progresstree/ProgressTreeView.h

@@ -30,6 +30,8 @@ public:
     explicit ProgressTreeView( QWidget* parent = 0 );
     virtual ~ProgressTreeView();
 
+    void setModel( QAbstractItemModel* model ) override;
+
 private:
     static ProgressTreeView* s_instance;
 };