ProvideValueTarget returns incorrect(?) TargetObject when nested markup extension is used on property

[miloszkukla]

As in title. I'm not sure whether it's best to return current result or the owner of the property or null like WPF does[1]. Here is a test demonstrating the situation: https://gist.github.com/miloszkukla/871d2321cc9d038dac936a6030add689

[1] https://docs.microsoft.com/en-us/dotnet/api/system.windows.markup.iprovidevaluetarget?view=netframework-4.8

An example case where the values from IProvideValueTarget are null even if the service is available is if your markup extension is nested within another markup extension, such as a Binding. An implementation that relies on IProvideValueTarget should always check for null values if only for the purpose of throwing a specific exception, if that is your intended design.