Since 2011.1 (11.1.20111.2054 ) this unhandled exception occurs for a left click or can be triggered with this code:

xamWebRibbon1.IsMinimized = true

I guess it is related to a tab/contexttab without a Name/Id set...

Stacktrace:

[ArgumentNull_Generic]
Arguments: 
Debugging resource strings are unavailable. Often the key and arguments provide sufficient information to diagnose the problem. See http://go.microsoft.com/fwlink/?linkid=106663&Version=4.0.60531.0&File=mscorlib.dll&Key=ArgumentNull_Generic
Parameter name: key
at System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
at Infragistics.Controls.Menus.Primitives.RibbonTabControl.EnsureSelectedIndex()
at Infragistics.Controls.Menus.Primitives.RibbonTabControl.SelectedTabChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.RaisePropertyChangeNotifications(DependencyProperty dp, Object oldValue, Object newValue)
at System.Windows.DependencyObject.UpdateEffectiveValue(DependencyProperty property, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, ValueOperation operation)
at System.Windows.DependencyObject.SetValueInternal(DependencyProperty dp, Object value, Boolean allowReadOnlySet)
at Infragistics.Controls.Menus.Primitives.RibbonTabControl.InvalidateMinimizedState()
at Infragistics.Controls.Menus.Primitives.RibbonTabControl.Ribbon_PropertyChanged(Object sender, PropertyChangedEventArgs e)
at Infragistics.Controls.Menus.XamRibbon.OnPropertyChanged(String propName)
at Infragistics.Controls.Menus.XamRibbon.IsMinimizedChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.RaisePropertyChangeNotifications(DependencyProperty dp, Object oldValue, Object newValue)
at System.Windows.DependencyObject.UpdateEffectiveValue(DependencyProperty property, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, ValueOperation operation)
at System.Windows.DependencyObject.SetValueInternal(DependencyProperty dp, Object value, Boolean allowReadOnlySet)
at Infragistics.Controls.Menus.XamRibbonTabItem.OnMouseLeftButtonDown(MouseButtonEventArgs e)
at System.Windows.Controls.Control.OnMouseLeftButtonDown(Control ctrl, EventArgs e)