Issue: Desgin界面看不到plain text
Error:The following classes could not be instantiated:
解决方法：修改App theme =》Material
baseline：
Error:This view is not constrained. It only has designtime positions, so it will jump to (0,0) at runtime unless you add the constraints
检查创建的textview，editText区别
.Java 里的变量名和添加的组件是一一对应的，名字不能错。
