Crash when allocating ascendancy node in another ascendancy

[brather1ng]

• Create new build
• Select Occultist Ascendancy
• Click on the root node of the Elementalist Ascendancy

Exception: System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.HashSet`1.Enumerator.MoveNext()
   at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
   at System.Linq.Enumerable.<SelectManyIterator>d__17`2.MoveNext()
   at POESKillTree.Utils.ObservableSet`1.ExceptAndUnionWith(IEnumerable`1 toRemove, IEnumerable`1 toAdd) in C:\GitHub\PoESkillTree_Source\WPFSKillTree\Utils\ObservableSet.cs:line 131
   at POESKillTree.SkillTreeFiles.SkillTree.AllocateSkillNodes(IReadOnlyCollection`1 toAdd) in C:\GitHub\PoESkillTree_Source\WPFSKillTree\SkillTreeFiles\SkillTree.cs:line 781
   at POESKillTree.Views.MainWindow.zbSkillTreeBackground_Click(Object sender, RoutedEventArgs e) in C:\GitHub\PoESkillTree_Source\WPFSKillTree\Views\MainWindow.xaml.cs:line 1468