ui: Intermediate fix to avoid early QWidget deletion due to smart pointers

TODO: get rid of smart pointers for Qt classes in QtGraphView

src/lib_gui/qt/view/QtGraphView.cpp

@@ -356,6 +356,9 @@ void QtGraphView::clear()
 		m_oldActiveNode.reset();
 		m_activeNodes.clear();
 
+		m_oldOldNodes = m_oldNodes;
+		m_oldOldEdges = m_oldEdges;
+
 		m_nodes.clear();
 		m_edges.clear();
 
@@ -777,6 +780,9 @@ void QtGraphView::switchToNewGraphData()
 		edge->setParentItem(nullptr);
 	}
 
+	m_oldOldNodes = m_oldNodes;
+	m_oldOldEdges = m_oldEdges;
+
 	m_oldNodes = m_nodes;
 	m_oldEdges = m_edges;