TimePicker with TextFieldAssist.HasClearButton attached property

[jbooth88]

Bug explanation

MaterialDesignThemes 4.7.1

Setting TextFieldAssist.HasClearButton="True" on the TimePicker control will make the clear button visible as expected, but clicking this button appears to have no effect. Expected behaviour is that the SelectedTime property will be set to null

<md:TimePicker
    xmlns:md="http://materialdesigninxaml.net/winfx/xaml/themes"
    md:HintAssist.Hint="Time"
    md:TextFieldAssist.HasClearButton="True"
    SelectedTime="{Binding TestTime, Mode=TwoWay, UpdateSourceTrigger=LostFocus, TargetNullValue=''}"
    Style="{StaticResource MaterialDesignFloatingHintTimePicker}"
    />

TestTime property is of type DateTime?

Version

4.7.1