x:bind nullreferenceexception when using the controltemplate :     When i'm writing a myButton,which is inherit from button, i set the ControlTemplate using an image,the source is x:bind to a string property in codebehind,like the screenshot below:

  [1]: https://i.stack.imgur.com/YN0DX.png

why there is an error "Object reference not set to an instance of an object ", i can't build project,how to fix it ,thanks! forgive my bad English.