PasswordBox binding error

[nabeelio]

I can't get the little show/hide icon to show up consistently - it might the first time you click into the box, but then if you click again, it won't show up again. This error shows up:

System.Windows.Data Warning: 4 : Cannot find source for binding 
with reference 'ElementName=TextBox'. BindingExpression:Path=IsSelectionActive; DataItem=null; 
target element is 'PasswordBox' (Name='ApiKey'); target property is 'NoTarget' (type 'Object')

The XAML is:

<PasswordBox
    x:Name="ApiKey"
    ui:ControlHelper.Header="Your API Key"
    TabIndex="3"
    PasswordChanged="ApiKey_PasswordChanged">
</PasswordBox>

And my App.xaml resources section:

<prism:PrismApplication.Resources>
    <ResourceDictionary>

        <ResourceDictionary.MergedDictionaries>
            <ui:ThemeResources CanBeAccessedAcrossThreads="True" />
            <ui:XamlControlsResources />

            <!-- Local custom resources -->
            <ResourceDictionary Source="/Resources/Icons.xaml" />
            <ResourceDictionary Source="/Resources/Styles.xaml" />
        </ResourceDictionary.MergedDictionaries>

        <BooleanToVisibilityConverter x:Key="BooleanToVisibiltyConverter" />
    </ResourceDictionary>
</prism:PrismApplication.Resources>