ActivityIndicator is not coming in proper position

Hi,

Please help me out how i will bring the activityIndicator in proper Position
    In all Platform it is not coming in proper position.It is coming in top not in center of the device.


    <ScrollView>
            <Grid BackgroundColor="#3F51B5">
                <Grid.RowDefinitions>
                  <RowDefinition Height="auto"></RowDefinition>
                  <RowDefinition Height="auto"></RowDefinition>
                  <RowDefinition Height="auto"></RowDefinition>
                  <RowDefinition Height="auto"></RowDefinition>
                </Grid.RowDefinitions>

                <StackLayout Padding="20" Grid.Column="0" Grid.Row="0"  >
                  <Image Source="panel.jpg"/>
                </StackLayout>
                <StackLayout Padding="20" Grid.Column="0" Grid.Row="1">
                  <Entry />
                </StackLayout>
                <StackLayout Padding="20" Grid.Column="0" Grid.Row="2">
                  <Entry />
                </StackLayout>
                <StackLayout  Padding="20" HorizontalOptions="FillAndExpand" Grid.Column="0" Grid.Row="3">
                  <Label/>
                </StackLayout>
                <ActivityIndicator x:Name ="LoadingLogin" IsRunning="False" IsVisible="False" Color="Black" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand"/>

            </Grid>
        </ScrollView>

Thanx in Advance