I am writing an wpf application with multiple skins. The code to switch theme is as below:

try
{
    Application.Current.Resources.MergedDictionaries.Add( resource );
}
catch( Exception ex )
{

}

The first time the code is called (to switch to a new theme), it is executed successfully; however, any subsequent calls to the same code would throw System.NullReferenceException.

The way I set up my themes are similar to what described here: http://www.codewrecks.com/blog/index.php/2008/05/22/simple-skinnable-and-theme-management-in-wpf-user-interface/.

Has anyone seen this error before? How do I go about debugging this since the exception is thrown else where? Any help is greatly appreciated.

StackTrace:
       at System.Windows.EffectiveValueEntry.GetFlattenedEntry(RequestFlags requests)
       at System.Windows.DependencyObject.EvaluateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry newEntry, OperationType operationType)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.StyleHelper.ApplyStyleOrTemplateValue(FrameworkObject fo, DependencyProperty dp)
       at System.Windows.StyleHelper.InvalidateContainerDependents(DependencyObject container, FrugalStructList`1& exclusionContainerDependents, FrugalStructList`1& oldContainerDependents, FrugalStructList`1& newContainerDependents)
       at System.Windows.StyleHelper.DoStyleInvalidations(FrameworkElement fe, FrameworkContentElement fce, Style oldStyle, Style newStyle)
       at System.Windows.StyleHelper.UpdateStyleCache(FrameworkElement fe, FrameworkContentElement fce, Style oldStyle, Style newStyle, Style& styleCache)
       at System.Windows.FrameworkElement.OnStyleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp)
       at System.Windows.FrameworkElement.UpdateStyleProperty()
       at System.Windows.TreeWalkHelper.InvalidateStyleAndReferences(DependencyObject d, ResourcesChangeInfo info, Boolean containsTypeOfKey)
       at System.Windows.TreeWalkHelper.OnResourcesChanged(DependencyObject d, ResourcesChangeInfo info, Boolean raiseResourceChangedEvent)
       at System.Windows.TreeWalkHelper.OnResourcesChangedCallback(DependencyObject d, ResourcesChangeInfo info)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1.StartWalk(DependencyObject startNode, Boolean skipStartNode)
       at System.Windows.TreeWalkHelper.InvalidateOnResourcesChange(FrameworkElement fe, FrameworkContentElement fce, ResourcesChangeInfo info)
       at System.Windows.Application.InvalidateResourceReferenceOnWindowCollection(WindowCollection wc, ResourcesChangeInfo info)
       at System.Windows.ResourceDictionary.NotifyOwners(ResourcesChangeInfo info)
       at System.Windows.ResourceDictionary.OnMergedDictionariesChanged(Object sender, NotifyCollectionChangedEventArgs e)
       at System.Collections.ObjectModel.ObservableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
       at System.Collections.ObjectModel.ObservableCollection`1.InsertItem(Int32 index, T item)
       at System.Windows.ResourceDictionaryCollection.InsertItem(Int32 index, ResourceDictionary item)
       at System.Collections.ObjectModel.Collection`1.Add(T item)