Android Studio預設主題AppTheme不顯示預覽畫面
設計頁面右上角有個紅色圓圈，裡面一個感嘆號，點開它檢視錯誤資訊：
The following classes could not be instantiated:
- android.support.v7.widget.ActionBarContainer (Open Class, Show Exception, Clear Cache)
- android.support.v7.widget.ActionBarContextView (Open Class, Show Exception, Clear Cache)
- android.support.v7.app.WindowDecorActionBar (Open Class, Show Exception, Clear Cache)
Tip: Use View.isInEditMode() in your custom views to skip code or show sample data when shown in the IDE.
If this is an unexpected error you can also try to build the project, then manually refresh the layout.
Exception Details
java.lang.ClassNotFoundException: android.view.View$OnUnhandledKeyEventListener
這是由於外掛版本與SDK版本不符引起的，解決辦法是開啟Project-%工程名%-app-build.gradle，修改這幾行:
compileSdkVersion 28 改為compileSdkVersion 27
targetSdkVersion 28 改為targetSdkVersion 27
implementation 'com.android.support:appcompat-v7:28.0.0-alpha3'改為
implementation 'com.android.support:appcompat-v7:27.1.1'
然後點選右上角的Sync Now進行同步
同步完成，構建完成後回到設計介面，預覽圖可以正常顯示。