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NRE in TooltipService

See original GitHub issue

Describe the bug Trying to set a tooltip for a TextBlock that is being destroyed/recycled ends up throwing a NRE.

To Reproduce It’s not easy to reproduce but maybe it’s easier to go through the implementation and protect the NRE exception. To reproduce, it needs to set a tooltip while the TextBlock is being destroyed/recycled in a virtualized list.

Expected behavior No NRE exception is thrown.

CallStack

Object reference not set to an instance of an object.
   at Avalonia.Controls.ToolTipService.TipChanged(AvaloniaPropertyChangedEventArgs e) in /_/src/Avalonia.Controls/ToolTipService.cs:line 25
   at System.Reactive.Subjects.Subject`1.OnNext(T value) in /_/Rx.NET/Source/src/System.Reactive/Subjects/Subject.cs:line 149
   at Avalonia.AvaloniaObject.RaisePropertyChanged[T](AvaloniaPropertyChangedEventArgs`1 change) in /_/src/Avalonia.Base/AvaloniaObject.cs:line 807
   at Avalonia.AvaloniaObject.ValueChanged[T](AvaloniaPropertyChangedEventArgs`1 change) in /_/src/Avalonia.Base/AvaloniaObject.cs:line 564
   at Avalonia.ValueStore.NotifyValueChanged[T](AvaloniaProperty`1 property, Optional`1 oldValue, BindingValue`1 newValue, BindingPriority priority) in /_/src/Avalonia.Base/ValueStore.cs:line 365
   at Avalonia.ValueStore.SetExisting[T](Object slot, StyledPropertyBase`1 property, T value, BindingPriority priority) in /_/src/Avalonia.Base/ValueStore.cs:line 265
   at Avalonia.ValueStore.SetValue[T](StyledPropertyBase`1 property, T value, BindingPriority priority) in /_/src/Avalonia.Base/ValueStore.cs:line 99
   at Avalonia.Controls.ToolTip.SetTip(Control element, Object value) in /_/src/Avalonia.Controls/ToolTip.cs:line 99
   at AppUiFramework.Controls.TextBlockAutoTooltip.SetupTooltipVisibility(TextLayout layout)
   at AppUiFramework.Controls.TextBlockAutoTooltip.CreateTextLayout(String text)
   at Avalonia.Controls.TextBlock.MeasureOverride(Size availableSize) in /_/src/Avalonia.Controls/TextBlock.cs:line 709

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