Android6.0文件提供程序在FileProvider.getUriForFile上不工作空指针

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

public class MyFileProvider extends FileProvider {
}

<provider
    android:name=".MyFileProvider"
    android:authorities="${applicationId}.myfileprovider"
    android:exported="false"
    android:grantUriPermissions="true">
    <meta-data
        android:name="android.support.FILE_PROVIDER_PATHS"
        android:resource="@xml/file_paths" />
</provider>

<paths xmlns:android="http://schemas.android.com/apk/res/android">
    <external-path name="external_files" path="." />
</paths>

File file = new File(Environment.getExternalStorageDirectory(), "example.jpg");
Uri fileUri = FileProvider.getUriForFile(context, context.getPackageName() + ".myfileprovider", file);