问题描述：对接天地图底图已经能够正常使用，现在想要添加矢量注记，参考的代码是示例代码中的MapOverlay，没有成功，报错如下：
FATAL EXCEPTION: main
Process: com.example.tianditu, PID: 18479
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.tianditu/com.example.tianditu.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.supermap.data.Datasets com.supermap.data.Datasource.getDatasets()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3771)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3938)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2318)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loopOnce(Looper.java:210)
at android.os.Looper.loop(Looper.java:299)
at android.app.ActivityThread.main(ActivityThread.java:8306)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:576)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1073)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.supermap.data.Datasets com.supermap.data.Datasource.getDatasets()' on a null object reference
at com.example.tianditu.MainActivity.onCreate(MainActivity.java:73)
at android.app.Activity.performCreate(Activity.java:8194)
at android.app.Activity.performCreate(Activity.java:8160)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1329)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3744)
MainActivity中代码如下
