Error when expanding an object in UE explorer

[m3cviz]

When I try to expand a class in UE explorer I get this error:
Stack Trace:
Thrown by:InitNodes
at UELib.Core.UObject.InitNodes(TreeNode node)
at UELib.Core.UStruct.InitNodes(TreeNode node)
at UELib.Core.UState.InitNodes(TreeNode node)
at UELib.Core.UObject.InitializeNodes(TreeNode node)
at UEExplorer.UI.Tabs.UC_PackageExplorer._OnClassesNodeExpand(Object sender, TreeViewCancelEventArgs e)

Exception occurred while initialising this node:
Object reference not set to an instance of an object.