Object reference not set to an instance of an object

Hi there. I receive an error (the error in the title) and I do not understand where this error is coming from.

<StackLayout Padding="20" Spacing="50">
        <Label x:Name="label"
               Text="Sample Text"
               FontSize="16"/>

        <Picker Title="Font Size">
            <Picker.Items>
                <x:String>Font Size = 8</x:String>
                <x:String>Font Size = 10</x:String>
                <x:String>Font Size = 12</x:String>
                <x:String>Font Size = 14</x:String>
                <x:String>Font Size = 16</x:String>
                <x:String>Font Size = 20</x:String>
                <x:String>Font Size = 24</x:String>
                <x:String>Font Size = 30</x:String>
            </Picker.Items>

            <Picker.SelectedIndex>
                <Binding Source="{x:Reference label}"
                         Path="FontSize">
                    <Binding.Converter>
                        <toolkit:ObjectToIndexConverter x:TypeArguments="x:Double">
                            <x:Double>8</x:Double>
                            <x:Double>10</x:Double>
                            <x:Double>12</x:Double>
                            <x:Double>14</x:Double>
                            <x:Double>16</x:Double>
                            <x:Double>20</x:Double>
                            <x:Double>24</x:Double>
                            <x:Double>30</x:Double>
                        </toolkit:ObjectToIndexConverter>
                    </Binding.Converter>
                </Binding>
            </Picker.SelectedIndex>
        </Picker>
    </StackLayout>