ProjectTree: Fix soft assert

Fix soft assert when accessing the lone project file node in a project
that failed to parse.

Change-Id: I73d9b0012292dd5516a0a9c81cdc4003eebf2a92
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/projectexplorer/projectnodes.cpp

@@ -908,4 +908,10 @@ ProjectNode *ContainerNode::rootProjectNode() const
     return m_project->rootProjectNode();
 }
 
+void ContainerNode::removeAllChildren()
+{
+    qDeleteAll(m_nodes);
+    m_nodes.clear();
+}
+
 } // namespace ProjectExplorer