MaterialButton: XPE when loading styles with triggers

通过将所示的 MaterialButton 样式添加到 App.xaml 并在 iOS 或 Android 上启动应用来复现该故障。然后检查 GetResource 和 Color.Secondary 附近的 Material.cs，以及其静态初始化器附近的 UI/MaterialButton.cs；当 App.InitializeComponent() 在没有报告的 XamlParseException 的情况下完成时，即表示完成。

🔙 Regression

In our Application we use MaterialButtons to emulate toggle buttons. The default ButtonType is Text.
For Buttons that are toggleable Buttons the ButtonType is set via a Binding and a custom BoolToButtonTypeConverter.
(false->Text, true->Flat)

To invert the TextColor when the Button is toggled on, we use the following style, defined in App.xaml.

<Style TargetType="ui:MaterialButton" ApplyToDerivedTypes="False">
    <Style.Triggers>
        <Trigger TargetType="ui:MaterialButton" Property="ButtonType" Value="Flat">
            <Trigger.Setters>
                <Setter Property="TextColor" Value="White"/>
            </Trigger.Setters>
        </Trigger>
    </Style.Triggers>
</Style>

Old (and correct) behavior

The app is initialized without any errors, Buttons behave as expected. (Xamarin.Forms 4.7.0.1142; XF.Material 1.5.8)

Current behavior

The app fails to start with a XamlParseException, as soon as the App-Constructor calls InitializeComponent(). (Xamarin.Forms 5.0.0.1874; XF.Material 1.7.7)
Callstack:

Xamarin.Forms.Xaml.XamlParseException: Position 62:53. Type converter failed: The type initializer for 'XF.Material.Forms.UI.MaterialButton' threw an exception. ---> System.TypeInitializationException: The type initializer for 'XF.Material.Forms.UI.MaterialButton' threw an exception. ---> System.NullReferenceException: Object reference not set to an instance of an object
  at XF.Material.Forms.Material.GetResource[T] (System.String key) [0x00000] in C:\Users\mhvdi\Documents\OpenSource\XF-Material-Library\XF.Material\Material.cs:57 
  at XF.Material.Forms.Material+Color.GetSecondaryColor () [0x00000] in C:\Users\mhvdi\Documents\OpenSource\XF-Material-Library\XF.Material\Material.cs:203 
  at XF.Material.Forms.Material+Color.get_Secondary () [0x00000] in C:\Users\mhvdi\Documents\OpenSource\XF-Material-Library\XF.Material\Material.cs:198 
  at XF.Material.Forms.UI.MaterialButton..cctor () [0x0003e] in C:\Users\mhvdi\Documents\OpenSource\XF-Material-Library\XF.Material\UI\MaterialButton.cs:20 
   --- End of inner exception stack trace ---
  at (wrapper managed-to-native) System.Reflection.RuntimeFieldInfo.GetValueInternal(System.Reflection.RuntimeFieldInfo,object)
  at System.Reflection.RuntimeFieldInfo.GetValue (System.Object obj) [0x0005e] in /Library/Frameworks/Xamarin.iOS.framework/Versions/Current/src/Xamarin.iOS/mcs/class/corlib/System.Reflection/RuntimeFieldInfo.cs:251 
  at Xamarin.Forms.BindablePropertyConverter.ConvertFrom (System.Type type, System.String propertyName, System.Xml.IXmlLineInfo lineinfo) [0x00055] in D:\a\1\s\Xamarin.Forms.Core\BindablePropertyConverter.cs:99 
  at Xamarin.Forms.BindablePropertyConverter.Xamarin.Forms.IExtendedTypeConverter.ConvertFromInvariantString (System.String value, System.IServiceProvider serviceProvider) [0x00169] in D:\a\1\s\Xamarin.Forms.Core\BindablePropertyConverter.cs:60 
  at Xamarin.Forms.Xaml.TypeConversionExtensions.ConvertTo (System.Object value, System.Type toType, System.Func`1[TResult] getConverter, System.IServiceProvider serviceProvider, System.Exception& exception) [0x00041] in D:\a\1\s\Xamarin.Forms.Core\Xaml\TypeConversionExtensions.cs:134 
   --- End of inner exception stack trace ---
  at Xamarin.Forms.Xaml.ApplyPropertiesVisitor.SetPropertyValue (System.Object xamlelement, Xamarin.Forms.Xaml.XmlName propertyName, System.Object value, System.Object rootElement, Xamarin.Forms.Xaml.INode node, Xamarin.Forms.Xaml.HydrationContext context, System.Xml.IXmlLineInfo lineInfo) [0x0007d] in D:\a\1\s\Xamarin.Forms.Xaml\ApplyPropertiesVisitor.cs:366 
  at Xamarin.Forms.Xaml.ApplyPropertiesVisitor.Visit (Xamarin.Forms.Xaml.ValueNode node, Xamarin.Forms.Xaml.INode parentNode) [0x0007b] in D:\a\1\s\Xamarin.Forms.Xaml\ApplyPropertiesVisitor.cs:60 
  at Xamarin.Forms.Xaml.ValueNode.Accept (Xamarin.Forms.Xaml.IXamlNodeVisitor visitor, Xamarin.Forms.Xaml.INode parentNode) [0x00000] in D:\a\1\s\Xamarin.Forms.Xaml\XamlNode.cs:91 
  at Xamarin.Forms.Xaml.ElementNode.Accept (Xamarin.Forms.Xaml.IXamlNodeVisitor visitor, Xamarin.Forms.Xaml.INode parentNode) [0x00044] in D:\a\1\s\Xamarin.Forms.Xaml\XamlNode.cs:152 
  at Xamarin.Forms.Xaml.ElementNode.Accept (Xamarin.Forms.Xaml.IXamlNodeVisitor visitor, Xamarin.Forms.Xaml.INode parentNode) [0x00044] in D:\a\1\s\Xamarin.Forms.Xaml\XamlNode.cs:152 
  at Xamarin.Forms.Xaml.FillResourceDictionariesVisitor.Visit (Xamarin.Forms.Xaml.ElementNode node, Xamarin.Forms.Xaml.INode parentNode) [0x000ec] in D:\a\1\s\Xamarin.Forms.Xaml\FillResourceDictionariesVisitor.cs:57 
  at Xamarin.Forms.Xaml.ElementNode.Accept (Xamarin.Forms.Xaml.IXamlNodeVisitor visitor, Xamarin.Forms.Xaml.INode parentNode) [0x00012] in D:\a\1\s\Xamarin.Forms.Xaml\XamlNode.cs:147 
  at Xamarin.Forms.Xaml.RootNode.Accept (Xamarin.Forms.Xaml.IXamlNodeVisitor visitor, Xamarin.Forms.Xaml.INode parentNode) [0x00079] in D:\a\1\s\Xamarin.Forms.Xaml\XamlNode.cs:211 
  at Xamarin.Forms.Xaml.XamlLoader.Visit (Xamarin.Forms.Xaml.RootNode rootnode, Xamarin.Forms.Xaml.HydrationContext visitorContext, System.Boolean useDesignProperties) [0x0007e] in D:\a\1\s\Xamarin.Forms.Xaml\XamlLoader.cs:235 
  at Xamarin.Forms.Xaml.XamlLoader.Load (System.Object view, System.String xaml, System.Reflection.Assembly rootAssembly, System.Boolean useDesignProperties) [0x000a2] in D:\a\1\s\Xamarin.Forms.Xaml\XamlLoader.cs:104 
  at Xamarin.Forms.Xaml.XamlLoader.Load (System.Object view, System.String xaml, System.Reflection.Assembly rootAssembly) [0x00000] in D:\a\1\s\Xamarin.Forms.Xaml\XamlLoader.cs:78 
  at Xamarin.Forms.Xaml.Extensions.LoadFromXaml[TXaml] (TXaml view, System.String xaml, System.Reflection.Assembly rootAssembly) [0x00000] in D:\a\1\s\Xamarin.Forms.Xaml\ViewExtensions.cs:49 
  at Xamarin.Forms.Xaml.ResourcesLoader.CreateFromResource[T] (System.String resourcePath, System.Reflection.Assembly assembly, System.Xml.IXmlLineInfo lineInfo) [0x000b1] in D:\a\1\s\Xamarin.Forms.Xaml\ResourcesLoader.cs:39 
  at Xamarin.Forms.ResourceDictionary.SetAndLoadSource (System.Uri value, System.String resourcePath, System.Reflection.Assembly assembly, System.Xml.IXmlLineInfo lineInfo) [0x00063] in D:\a\1\s\Xamarin.Forms.Core\ResourceDictionary.cs:76 
  at Vendor.MobileApp.App.InitializeComponent ()

Reproduction steps

Add the style above into App.xaml and try to launch the App in an Emulator.

Configuration

Version: 1.7.7

Platform:

• 📱 iOS
• 🤖 Android
• 🏁 WPF
• 🌎 UWP
• 🍎 MacOS
• 📺 tvOS
• 🐒 Xamarin.Forms