Oh, it's my fault for providing incomplete information to you guy.
I am binding Tap Command to SfListview so when any item is tapped it will open SfPopup and pass the tapped item's data to Sfpopup for display.
And for that script, i think if GroupHeader is tapped it's will count as item tapped and doesn't have any data to transmit over SfPopup so it will crash.
My XAML:
<SfListView ItemsSource={Binding ProductList}
IsStickyGroupHeader="True"
TapCommand="{Binding ItemTapCommand}" >
<SfListView.DataSource>
<DataSource>
<DataSource.GroupDescriptors>
<GroupDescriptor PropertyName="Type"/>
</DataSource.GroupDescriptors>
</DataSource>
</SfListView.DataSource>
<SfListView.GroupHeaderTemplate>
<DataTemplate>
<ViewCell>
<ViewCell.View>
<StackLayout BackgroundColor="WhiteSmoke">
<Label Text="{Binding Key}"
FontSize="20"
TextColor="Black"
FontAttributes="Bold"
VerticalOptions="Center"
HorizontalOptions="Start"
Padding="15,7,0,7" />
</StackLayout>
</ViewCell.View>
</ViewCell>
</DataTemplate>
</SfListView.GroupHeaderTemplate>
<SfListView.ItemTemplate>
.........
</SfListView.ItemTemplate>
</SfListView>
<SfPopupLayout IsOpen="{Binding PopupIsOpen}">
<SfPopupLayout.PopupView>
<PopupView>
........
</PopupView>
</SfPopupLayout.PopupView>
</SfPopupLayout>