尝试调用虚方法：nullpointerexception

Question

错误堆栈跟踪如下。

我是否需要在build.gradle文件中为sqlite数据库添加依赖项

java.lang.NullPointerException: Attempt to invoke virtual method 'long android.database.sqlite.SQLiteDatabase.insert(java.lang.String, java.lang.String, android.content.ContentValues)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2892)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3027)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:101)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:73)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1786)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'long android.database.sqlite.SQLiteDatabase.insert(java.lang.String, java.lang.String, android.content.ContentValues)' on a null object reference
        at crystals.com.riddles.DbHelper.addQuestion(DbHelper.java:86)
        at crystals.com.riddles.DbHelper.onCreate(DbHelper.java:36)
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:368)
        at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:297)
        at crystals.com.riddles.DbHelper.getAllQuestions(DbHelper.java:92)
        at crystals.com.riddles.QuizActivity.onCreate(QuizActivity.java:27)
        at android.app.Activity.performCreate(Activity.java:7117)
        at android.app.Activity.performCreate(Activity.java:7108)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1262)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2867)