WPF文本框事件TextChanged PreviewTextInput不会触发

I have inherited an bigger C# Project from a college (hes gone so I can't ask him anymore). Im on VS2012 + .Net4.6 The first time in this Project I added a WPF Textbox to a nonmodal Window. The problem is: I cant change the text in the TextBox.I have inherited an bigger C# Project from a co