在我的应用程序中，我有三个 PNG 文件用作活动的背景，放置在文件夹 drawable-nodpi 中，但应用程序在我用于测试的平板电脑 Samsung Galaxy Tab 2 10.1 上崩溃并给了我这个错误：
日志猫：
E/AndroidRuntime(4846): FATAL EXCEPTION: main
E/AndroidRuntime(4846): java.lang.RuntimeException: Unable to start activity ComponentInfo{it.bisemanuDEV.mathTools/it.bisemanuDEV.mathTools.MainActivity}: java.lang.NullPointerException
E/AndroidRuntime(4846): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097)
E/AndroidRuntime(4846): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2122)
E/AndroidRuntime(4846): at android.app.ActivityThread.access$600(ActivityThread.java:140)
E/AndroidRuntime(4846): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1228)
E/AndroidRuntime(4846): at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(4846): at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime(4846): at android.app.ActivityThread.main(ActivityThread.java:4895)
E/AndroidRuntime(4846): at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(4846): at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime(4846): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:994)
E/AndroidRuntime(4846): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:761)
E/AndroidRuntime(4846): at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime(4846): Caused by: java.lang.NullPointerException
E/AndroidRuntime(4846): at it.bisemanuDEV.mathTools.MainActivity.onCreate(MainActivity.java:53)
E/AndroidRuntime(4846): at android.app.Activity.performCreate(Activity.java:5163)
E/AndroidRuntime(4846): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094)
E/AndroidRuntime(4846): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2061)
但该应用程序适用于不同型号的智能手机。可能是什么问题呢？