Xamarin Android后台定位服务null

Question

我使用this demo为我的Android项目使用后台定位服务（因为Geolocator在Android上不支持后台）。直到昨天它给我以下错误时，我才遇到任何问题：

[ServiceConnection] OnServiceConnected Called
Service Connected
[LocationService] You are about to get location updates via 
[Mono] Assembly Ref addref Mono.Security[0xa7dd6ba0] -> System[0xadd5aac0]: 15
[Mono] DllImport attempting to load: '/system/lib/liblog.so'.
[Mono] DllImport loaded library '/system/lib/liblog.so'.
[Mono] DllImport searching in: '/system/lib/liblog.so' ('/system/lib/liblog.so').
[Mono] Searching for '__android_log_print'.
[Mono] Probing '__android_log_print'.
[Mono] Found as '__android_log_print'.
[MonoDroid] UNHANDLED EXCEPTION:
[MonoDroid] Java.Lang.IllegalArgumentException: invalid provider: null
[MonoDroid]   at Java.Interop.JniEnvironment+InstanceMethods.CallVoidMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args) [0x00069] in <ab9e0fea7db841c1a1450c120052eb14>:0 
[MonoDroid]   at Java.Interop.JniPeerMembers+JniInstanceMethods.InvokeVirtualVoidMethod (System.String encodedMember, Java.Interop.IJavaPeerable self, Java.Interop.JniArgumentValue* parameters) [0x0002a] in <ab9e0fea7db841c1a1450c120052eb14>:0 
[MonoDroid]   at Android.Locations.LocationManager.RequestLocationUpdates (System.String provider, System.Int64 minTime, System.Single minDistance, Android.Locations.ILocationListener listener) [0x0007e] in <21f1b0b5ad7242d7afaeceae3e6dbac8>:0 
[MonoDroid]   at Divco.LocationService.StartLocationUpdates () [0x0003c] in /Users/ediewoelfle/Developer/ancient-bastion-mobile/Droid/Services/LocationService.cs:78 
[MonoDroid]   at Divco.LocationServiceConnection.OnServiceConnected (Android.Content.ComponentName name, Android.OS.IBinder service) [0x0004f] in /Users/ediewoelfle/Developer/ancient-bastion-mobile/Droid/Services/LocationServiceConnection.cs:42 
[MonoDroid]   at Android.Content.IServiceConnectionInvoker.n_OnServiceConnected_Landroid_content_ComponentName_Landroid_os_IBinder_ (System.IntPtr jnienv, System.IntPtr native__this, System.IntPtr native_name, System.IntPtr native_service) [0x00019] in <21f1b0b5ad7242d7afaeceae3e6dbac8>:0 
[MonoDroid]   at (wrapper dynamic-method) System.Object.31(intptr,intptr,intptr,intptr)
[MonoDroid]   --- End of managed Java.Lang.IllegalArgumentException stack trace ---
[MonoDroid] java.lang.IllegalArgumentException: invalid provider: null
[MonoDroid]     at android.location.LocationManager.checkProvider(LocationManager.java:2317)
[MonoDroid]     at android.location.LocationManager.requestLocationUpdates(LocationManager.java:490)
[MonoDroid]     at md587a0a65fe57ffee4ef3988209babfc12.LocationServiceConnection.n_onServiceConnected(Native Method)
[MonoDroid]     at md587a0a65fe57ffee4ef3988209babfc12.LocationServiceConnection.onServiceConnected(LocationServiceConnection.java:38)
[MonoDroid]     at android.app.LoadedApk$ServiceDispatcher.doConnected(LoadedApk.java:1535)
[MonoDroid]     at android.app.LoadedApk$ServiceDispatcher$RunConnection.run(LoadedApk.java:1563)
[MonoDroid]     at android.os.Handler.handleCallback(Handler.java:751)
[MonoDroid]     at android.os.Handler.dispatchMessage(Handler.java:95)
[MonoDroid]     at android.os.Looper.loop(Looper.java:154)
[MonoDroid]     at android.app.ActivityThread.main(ActivityThread.java:6316)
[MonoDroid]     at java.lang.reflect.Method.invoke(Native Method)
[MonoDroid]     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)
[MonoDroid]     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="42" android:versionName="0.1" package="com.divcodelivery.divco">
<uses-sdk android:minSdkVersion="23" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.CALL_PHONE" />
<uses-permission android:name="android.permission.SEND_SMS" />
<uses-permission android:name="android.permission.ACCESS_LOCATION_EXTRA_COMMANDS" />
<uses-permission android:name="android.permission.ACCESS_MOCK_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_SURFACE_FLINGER" />
<uses-permission android:name="android.permission.WAKE_LOCK" />
<uses-permission android:name="android.permission.READ_PHONE_STATE" />
<application android:label="Divco" android:icon="@drawable/logo_size_icon">
    <provider android:name="android.support.v4.content.FileProvider" android:authorities="com.divcodelivery.divco.fileprovider" android:exported="false" android:grantUriPermissions="true">
        <meta-data android:name="android.support.FILE_PROVIDER_PATHS" android:resource="@xml/file_paths"></meta-data>
    </provider>
</application>
</manifest>

我已经找到了null locationProvider，但是，我不确定为什么。