org. xml. sax. SAXParseException； lineNumber: 141； columnNumber: 252； cvc-complex-type. 2.4. a: Found element 'B

background ：

org.xml.sax.SAXParseException; lineNumber: 141; columnNumber: 252; cvc-complex-type.2.4.a:  Found the element  'base-extension'  Invalid content at the beginning . Should use  '{layoutlib}'  One start .

Solution ：

The problem arises , It is generally a problem in layout , The following error code ：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <Button
        android:id="@+id/btn_test"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text=" test "
        ></Button>

</Lin